Gun follows the story of Dr. Brown, who defeated a mad android known as Golden Silver, which had tried to suck out the energy of planet G-9. Dr. Brown extracted four Mystical Gems, sealing them on one of the planet's moons, guarded by the four Gunstar siblings. Years later, a dictator named Smash Daisaku kidnapped Gunstar Green and brainwashed him into revealing the location of the Mystical Gems. The dictator is now close to reactivating Golden Silver, prompting the Gunstar twins Red and Blue, along with their sister Yellow, to embark on a mission to stop Smash Daisaku.
Gun is a fast-paced side-scrolling shooter where players control either Red or Blue Gunstar. The player has access to four basic weapons: Force, rapid shot; Lightning, a straight shot that penetrates enemies; Chaser, a homing weapon; and Flame, a powerful close-ranged. Players can combine two weapons to produce a new one, resulting in a total of fourteen weapons. Melee attacks and moves such as grabbing and tossing enemies, jumping, and sliding are also available.