Geff's exit is one of the rooms in Aeternis.
Layout[ | ]
This sealed hatch leads outside Aeternis. The lock on the door can be disengaged with the Holy Grail. This is the good ending available from the tunnel junction. You are met by Geffrye, but be warned, once you approach the exit, Tobias will appear and attack Geff, taking back the stolen grailstone and leaving your ally to crumble to dust. You are free to leave.