The G.E.C.K. in-game is located in Vault 87. To get it you can 1. Allow the friendly Super Mutant, Fawkes, found imprisoned in V87, to retrieve it for you. 2. If you have Charon as a follower you can send him into the chamber to get it. Or 3. Pop some Rad-X put on a radiation suit and run in and get it yourself. If you choose #3, go into the chamber, turn right then left. From here there is a room on your left with a large silver cylinder, this is the GECK containment, activate it, it will open then you are free to take the GECK and leave.