Gunstar Heroes is a run and gun game played from a side-scrolling perspective similar to Contra.[3] The game can be played in single-player, or cooperatively with a partner. The players take on the role of Gunstar Red and Gunstar Blue as they battle with an evil empire for control over a set of powerful gems.[4] The game features seven stages, of which the first four can be tackled in any order.[4] The stage formats vary; while some feature a typical left-to-right format, others have the player riding in a mine cart along walls, fighting enemies on a helicopter, or playing a board game.[4][5] Completing a level grants the player an extension to their maximum health.[4]
When starting a game, the player can choose either a free or fixed firing stance; the fixed stance immobilizes the character when shooting, while the free stance has the player move in the direction they are firing.[5] The player also has a choice of starting weapon. There are four shot types in the game: a homing shot, lightning blaster, flamethrower, and machine gun. Each weapon has its strengths and weaknesses, and can be swapped with others from item drops in each stage.[6] The weapons can be combined with each other to produce unique shot types. For example, the homing shot can be combined with the machine gun to add a homing effect to the latter, or two lightning shots can be combined to create a more powerful lightning gun.[6] In addition to firing their weapon, the player characters can pull off a series of acrobatic maneuvers including jumping, sliding, and grabbing and throwing enemies.[4]