After finishing the Gestalt battle with Bahamuth Eidolith, a series of epic cut scenes will play out. Once you spawn at camp, the achievement will pop. Get ready for an open world experience from here on out!
For those having issues with Bahamuth Eidolith, his gestalt meter will rise when defending as SEN or weakening with SAB. Make sure you set some Paradigms for this. You can always "retry" the battle and set them before walking back into the fight.