In the 21st century Earth is at war with the Ramulons, an alien race determined to prevent Earth from expanding further into space. With Earth being overwhelmed by the superior force of the Ramulons, it is up to a lone mech pilot to take the experimental "GMR-35 Saladin" through their lines and strike into the heart of the Ramulon army itself.
Gigantic Army is a fast-paced side-scrolling mech platform shooter, similar to Cybernator. You pilot a heavily armed yet mobile mech that can jump, hover, dash and deflect enemy attacks with a strong shield, which can however be destroyed if it takes too much battering. Your armaments consist of a main cannon weapon and a limited use special weapon, as well as a close-range strike with a high-power piledriver. At the start of the game you get a choice of which main and special weapons to equip, from a total of three of each.
Throughout the six levels you will come across containers that when broken open will reveal health, time or weapon powerup items. Grabbing a pickup when the number in question is at max will instead grant a hefty point bonus. If your health or time runs out, it's game over, though you have a handful of continues that lets you start at the beginning of your current level but without powerups and with a reset score. At the middle and end of each level you will encounter an extra-large robot enemy that you have to take down, typically with one or more weak points that have to be destroyed. After finishing a level your shield and special weapon is completely refilled, your health however only gets a limited refilling.
Initially, you have the choice of "easy" and "normal" difficulty settings, with two more being unlockable.