Gigantic Army is set in a world where Earth is fighting for survival against an alien force known as the Ramulons. With the enemy's superior numbers overwhelming our defenses, it falls to a lone mech pilot to take down their army and turn the tide of war.
Players control a heavily armed mech, able to jump, hover, dash, and deflect enemy attacks with its strong shield. However, this shield can be destroyed if it takes too much damage. The mech's arsenal consists of a main cannon weapon, a limited-use special weapon, and a powerful piledriver for close-range combat.
At the start of the game, players can choose from three options for their main and special weapons. Throughout the six levels, they'll encounter containers that can be broken open to reveal health, time, or power-up items. Grabbing these items when their value is maxed out will reward a significant score boost.
The gameplay involves navigating through each level while avoiding running out of health or time. If either condition is met, the game is over, unless a continue is used to restart from the beginning without power-ups. At the end of each level, players must take down an extra-large robot enemy by destroying its weak points.
Initially, players can choose from two difficulty settings: easy and normal. Two more difficulty levels become available as the player progresses through the game.