In the course of your adventure, you will encounter Eidolons, other wordly beings whose formdable powers can be harnessed for your cause. Eidolons will not bow to just anyone, however - would-be masters must prove their worthiness on the field of battle.
Eidolons may sustain damage from standard attacks like any other foe, but this alone will not lead you to victory. Players are encouraged to experiment with abilities to uncover and exploit the unique vulnerabilities that will bring each adversary to heel.
As you demonstrate your power to the Eidolon, its gestalt gauge will slowly increase. Continue to attack until the gauge is full, at which you may press the Square button to claim victory.
Characters who have claimed dominion over their personal Eidolon will learn the "Summon" technique. Summon an Eidolon in battle and it will fight at your side, replenshing your health and hurting your enemies until its SP gauge is depleted. The SP gauge will decrease gradually over time, or when the Eidolon takes damage.
Once an Eidolon has been summoned, you may press the Square button to enter gestalt mode. While gestalt mode may be activated at any time, it will remain in effect only as long as your gestalt gauge permits. Charge the gauge by executing attack chains with your Eidolon prior to engaging gestalt mode to maximize the time you will have to deal damage to your foes in battle.
Controls in gestalt mode differ from standard battle controls, using combinations of left analog stick motions and button presses to execute an array of impressive attacks. Autogestalt allows you to leave the choise to your Eidolon by pressing the X button if you find yourself at a loss for your next move.
Be aware that using abilities in gestalt mode will reduce your gestlat gauge by the amount displayed to the left of the ability name. When the gauge reaches zero, your Eidolon will be dismissed. Chains and staggers will be reset upon dismissal from gestalt mode, but in return, your party will return to the fray fully healed and cured of all ailments.