Gain Ground is a 1988 action-strategy game by Sega. It was originally produced for the Arcades, later ported/remade for the Sega Master System, the Sega Genesis, the TurboGrafx-16, the PlayStation 2, and even Japanese mobile phone platforms like i-Mode and Yahoo!Keitai. Among them, the Sega Genesis version is the nearest to original game, while adding one new Epoch^note Although, the PlayStation 2 version is just a 3D remake of the Arcade version, down to exact physics in some cases, with extra settings and gamemodes. The Genesis version has been ported several times since its debut release, including to the Virtual Console, Platform/Steam, the Nintendo eShop, and game collections across several other consoles, including even some Plug-N-Play collections. The Arcade version, meanwhile, was also released as part of the Sega Ages series on the Nintendo Switch.

The game set in the year 2348, where a long period of peace has deprived the earthlings of their instinct to wage war. The Federated Government, greatly concerned regarding this ever increasing dangerous situation, developed a Gain Ground simulation system in the year 2348 in an effort to instigate their ever waning fighting spirit. However, suddenly without warning, the Supercomputer went berserk and took many of the citizens as hostages. In order to rescue the people, three of the bravest warriors were urgently dispatched to go forth into the deadly Gain Ground.

The goal of each stage is to either get all characters you control out of the stage via the marked exit, or to kill all enemies on the map. The latter ends the level without having to get any more characters out, but it can be quite difficult depending on where the enemies are hiding. If the timer for the level runs out before all enemies are defeated, all characters in the player's party that did not make it into the exit are lost, and if all of a player's characters are killed before they can reach the exit of a level, the game is over.^note Unless a continue is used, which will reset the player's score to 0 and give him/her the default starting roster of three.

The game also has "Hostages" that can be rescued by picking them up and carrying them to the exit. Each hostage unlocks a new character^note starting in the next stage, of course, since characters that have reached the exit cannot be used until the next stage., which will have a unique set of weapons. However, if all of the enemies are defeated before a hostage is brought to the exit, the character will not be obtained. Also, any character that you lose turns into a hostage- allowing you to potentially pick them back up and carry them to the EXIT to recover them. However, only one player-made hostage can be on the screen at any given time — so if another character gets hit while that first lost character is still on the field, the older of the two disappears for good. This unique mechanic means that your lives are directly tied to the tactics you have to choose from — if you lose a character and cannot rescue them, you're down a life AND you've lost the weapon they had.

Several stages in Project × Zone are modeled after this game, and use a remix of its main theme.

This game provides examples of:

• Aerith and Bob: Some characters have very distinguished and unique names, such as "Zaemon", "Gascon", and "Mud Puppy". Others, however, can be as simple as "Johnny", "Betty", and "Honey".
• A.I. Is a Crapshoot: The Gain Ground system is the Big Bad, being a computer program that took the scientists who created it hostage, along with the several rescue teams that failed before yours came along.
• Alliterative Title: Gain Ground
• Ambushing Enemy: Several enemies in Gain Ground enter the level from off-screen when certain conditions are fulfilled, such as when a certain amount of time has passed, when a different on-screen enemy has been defeated, or when a player steps in a specific spot. All three of these can lead to ambushes, especially since the developers predicted ahead of time where the player(s) would likely be standing in any given level when the spawn conditions are met, and put the enemies nearby to give the player(s) less reaction time.
• Bombardier Mook: Grenade-throwing foes exist in the China-themed levels across all versions of the game, and in the Genesis version's Modern Era levels as well.
• Conveyor Belt o' Doom: A few instances across the franchise.
  • In the Sega Genesis version, Round 4 Stage 8 is a spiral-shaped path to the EXIT with conveyer belts that push the player further into the spiral, and in both cases, closer to the enemies trying to kill them.
  • In the Sega Master System version, there are several levels across several of the game's worlds which feature animated flashing-blue tiles. There are three types of these tiles: type one animates in a straight line, and push the player in the direction they're animating in. Type two animates in a spiral, forcing the player to spin around on the spot and thus making walking in a straight line a nightmare. And finally, type three are the ones that animate pulsing inward toward their center, and kill any player that walk over it. The lattermost of the three is often placed at the rear-end of a long section of any combination of the former two, forcing the player to both fight against the forces of the tiles and the enemies of the stage simultaneously.
• Denial of Diagonal Attack: Played with and averted. All characters can fire in all eight directions with their normal attacks, but some characters can only shoot in certain directions with their special attacks.
• Easy Level Trick: Zaemon's tornadoes spiral in a circular pattern and can hit multiple enemies. While using him in most cases is a matter of precise skill and/or random luck, his magic has one major exploitation: the tornado can hit the same enemy multiple times. Therefore, one can defeat multiple-hit enemies (such as the tank boss in the fourth round of the Genesis version) by simply firing a tornado close-range at them (or in the case of the tank, in it's path) and watching it bounce around the inside of their hit box until they die.
• Easy-Mode Mockery: Playing on easy mode in the Genesis version will cause the game to skip the entire fourth world.^note This world is the Modern Era, a world added to the Genesis version of the game. Arguably, it is the hardest world, since it has fast-firing and fast moving enemies, along with some tricky-to-save hostages. Furthermore, doing so cuts out half of the credits sequence, not allowing the player to view the comic-style cutscene at the end of the game with the actual developer credits in it, instead only seeing the cutscene and message about the Gain Ground System shutting down.
• Empty Room Until the Trap: The enemy count at the top of the screen tells you how many enemies are left alive in a level. This is crucial, as in multiple situations, enemies can charge in from off-screen and ambush you if you don't realize what's going on. An especially common occurrence in Round 1 Stage 3, where new players can get confused after they defeat everyone and the level doesn't end, resulting in them going for the exit and being charged down a narrow corridor by fast off-screen enemies from above.
• Gender Is No Object: Depends on the version.
  • Averted in most versions of Gain Ground, which have only four female characters compared to the sixteen other male (or otherwise ambiguously gendered) characters. To that effect, since weapons are restricted to each individual unit, the female characters only have boomerangs and grenades with sidearm pistols, and never outgrow those restrictions.
  • Played straight in the Sega Master System version. P1s characters are all male (and blue), and P2s characters are all female (and red). Thus, the gender of any given rescuable hostage is only decided after it joins a party, making there a male and female counterpart of each available unit/weapon type.
• Hitbox Dissonance: A few enemies, but none more notable than in the World 2 Boss.
  • The boss' lightning sword attack not only hits an area significantly larger than the thunderbolt itself, but also lingers for a bit after the attack appears to have ended.
  • There is also a gap between the hitbox of the spawned thunder and that of the boss itself which a player can stand between and be completely safe from harm while the boss can do nothing.
  • Using Mud Puppy, it is possible to fire a shot straight into the knights standing between the boss and the EXIT- despite the fact that the bullet flies right through the boss' shoulder to do so. Which is odd, considering how getting insomuch as within breathing distance of the boss' backside insta-kills you.
• Instant-Win Condition: The level ends instantly when everyone reaches the exit or when all enemies are killed. Note that killing the last enemy while escorting a hostage causes that hostage to be lost.
• Justified Extra Lives: Every life is its own character, with different attacks and range. You gain new lives by rescuing characters in the field (by touching them and then bringing both characters to the exit); if a character is killed, they'll be frozen on the map, and another character in your control can rescue them. (If a character dies while escorting another, though... the escortee will be lost!)
• Notice This:
  • Hostages blink in different colors, are not moving or animated at all (until you pick them up), and are much smaller than any other enemies in the game in order to indicate they are important things to collect and not mooks.
  • Exit(s) are giant yellow rectangles with the word EXIT written on them in bold text. Enemies refuse to stand on them, and they are almost always placed on a contrasting background, especially when they're anywhere but the entire top edge of the screen.
• One-Hit Kill:
  • Every one of the twenty different player characters goes down in one hit, no matter if it's a throwing knife, grenade, fireball, a futuristic laser, or just an enemy bumping into you.
  • Most enemies can be defeated in one shot, especially in the first round. In fact, some weapons are strong enough to take out enemies in one hit that would normally take multiple shots, most notably Mud Puppy's plasma cannon.
• One World Order: The manuals refer to the "United Earth Government" as the legal body that rules the world in the year 2348.
• Permadeath: The game's hard mode starts you with all characters. If one dies, they'll be Killed Off for Real!^note Assuming you can't save them after they turn into a hostage, that is.
• Point of No Continues: You can no longer use continues once you reach the Final Epoch; if all your characters die by that point, your game will be permanently over!
  • To make things even worse, no extra players can join in once the Final Epoch starts, meaning that they cannot cheat in an extra play for you that way either!
• Sand Worm: Appear in 3-1 and 3-2 of most versions of the game^note Arcade, Genesis, SX, and PlayStation 2, the lattermost of which lets one see its full body via its 3D model, burrowing in and out of the sand to dodge attacks. They spit either mini-scarab enemies that clog the battlefield, or various patterns of fireballs.
• Suddenly-Harmful Harmless Object: The Boss of World 1 acts this way — it only activates once the other four mini-bosses in the level are defeated, and cannot be interacted with or damaged in any way until it activates. And surprise — it has TWO heads, so it shoots fireballs twice as often as its mini-boss counterparts!