Characters who have obtained a summon will gain a new TP command, "Summon", and it costs 3 TP to activate. Upon choosing the command, a character’s summon will appear and heal all allies. The other two characters in the party will temporarily leave the battlefield, leaving the summon to fight alongside the summoner.
A summon has a certain amount of summon points, or SP for short. This bar depletes steadily over time and each time a summon takes damage (the summon has infinite HP). If the summoner is KOed while the summon is fighting, the summon will revive the summoner with full HP and then leave the battlefield.
After the summon has been called, a new gauge appears below the summoner’s HP display in the lower right of the screen - Gestalt gauge. You fill it up by fighting alongside the summon. From this moment on you can also press Square/X to activate Gestalt mode. However, it is advisable that you fight with the summon for a while, so that you can enter Gestalt mode with a higher amount of timer points.
Upon entering Gestalt mode, a timer points gauge appears in the lower left corner, with a list of abilities you can perform by pressing certain button combinations with the left analog stick. Each command deducts a certain amount of timer points. Pressing Triangle/Y will use up all timer points for an extra powerful finishing move, which also ends Gestalt mode.