Gigantic Army is a fast-paced side-scrolling mech platform shooter where you pilot a heavily armed yet mobile mech that can jump, hover, dash, and deflect enemy attacks with a strong shield. The goal is to take the experimental "GMR-35 Saladin" through the Ramulon army's lines and strike into their heart.
You start by choosing your main and special weapons from a total of three options each. As you progress through six levels, you'll encounter containers that can be broken open to reveal health, time, or weapon power-up items. Some pickups will grant bonus points if you collect them when the item count is maxed out.
Game over occurs when your health or time runs out, but you have a limited number of continues to restart from the beginning with no power-ups and reduced scores. At the end of each level, an extra-large robot enemy awaits, which can be taken down by targeting its weak points. After completing levels, your shield and special weapon are refilled, while health only gets a limited recharge.
The game initially offers two difficulty settings, with two more unlockable as you progress through the game.