Gigantic Army is a run-and-gun in a similar style to Metal Slug. You are in control of a mech and can select your main and special weapon loadout each from three options prior to starting the game. Each weapon has different advantages and disadvantages, with varying powers, rate of fires, and targeting systems.
The game follows a war between the humans and an alien race known as the Ramulons, and you are placed on the enemy homeworld in the front-line ground assault.
The game differs from other run-and-guns in that you have a health bar and shield (with its own health bar) that are depleted as you are damaged, but between stages you do not fully heal as the repair corps has limited resources.