The bulk of the fighting in Xenogears is done on Gears, your classic big armored robots. The battle system is totally different when you're on your Gears; there's no AP. Instead, you have Fuel. Unlike AP, Fuel is a lasting statistic -- every time you attack, your Fuel goes down, and if you run out of Fuel, you can't attack. You can restore Fuel by going to a Gear Shop, or by using the Charge command during battle (which only restores a minimal amount of fuel).

   When attacking in your Gears, you can use Triangle, Square, or X attacks, like on foot. You can only pick one, though, and each uses a different amount of Fuel -- Triangles use 10, Squares 20, and X attacks (the strongest, of course) consume 30 Fuel.

   Every time you attack on a Gear, your Attack Level increases, from 0 to 1 to 2 to 3, which enable to use Gear Deathblows. While at Attack Level 1, you can use any Deathblows starting with Triangle. Attack Level 2 enables to use Deathblows starting with Square OR Triangle, and Level 3 obviously lets you use Deathblows beginning with any of the three buttons. Once you execute a Deathblow, your Attack Level will reset back to 0. Also, note that you don't learn Gear Deathblows separately; you acquire them on a one-for-one basis as you get character Deathblows. You can also equip DeathBlower items to get additional Gear Deathblows.

   While on your Gears, you can usually use a few Ethers, though your selection is rather limited. They consume your character's EP as usual -- Gears don't have their own EP. Some Gears do have special Options that are basically Ethers that use Fuel.

   All Gears also have an ability called Booster, which you can activate from the menu. While Booster is on, your AT bar fills twice as fast, but you expend extra Fuel each turn. It's usually a waste of Fuel, but you'll need it in some battles.

   Finally, Gears don't increase in power as you gain experience (except Chu-Chu). Instead, you upgrade them by purchasing new Frames (for HP), Armor (for defense), and Engines (for attack power and Fuel capacity). Some Gears can also equip weapons, and all of them (besides Chu-Chu) can use accessories.