In Gain Ground, players control one of a set of characters at a time, each with different weapons. To beat a level, players must reach the exit point with at least one character or destroy all enemies on the level before time runs out. There are 40 levels in the arcade version of the game. The Master System and the Genesis/Mega Drive have 50 levels in the game.

Normal mode starts with three players. There are captive characters littered across all levels, which can be rescued by walking over, then escorting the controlled character to the exit point. If a player controlled character is killed, that character turns into a captive, except that they will disappear if the next active player controlled character dies, exits the level without them, or the player has no characters left in their party. In Hard mode, you start the game with all twenty characters, but all the captive characters are removed from the levels.

The game is over when all controlled characters in the party are killed without any reaching the exit. However, there are three continues which allow a player to restart the level with their original three characters.

The game consists out of four rounds, each having 10 stages, where stage 10 is a boss level. There are also ten completely new levels added to the Genesis/Mega Drive version, this Modern Epoch takes place in the streets of the city.