Retrieve the GECK for Arroyo

Retrieve the GECK for Arroyo is a quest in Fallout 2. It is a main quest, given to the Chosen One at the start of the game.

Detailed walkthrough
There are two GECKs in the game, one in Vault 13 and one at the Control Station ENCLAVE.

There are three ways to retrieve the G.E.C.K. from Vault 13:
 * Tell Sheriff Dumont in NCR Downtown that you're looking for "honest work." He will refer you to Westin (the entrance to his ranch is just to the west of the sheriff). The referral from the sheriff (must pass a Karma check) will get the guard to open the gate. Take the quest to investigate raids on the brahmin; if you have a 100% Outdoorsman skill, you can track the perpetrators (talking deathclaws) back to Vault 13. (Note that a possible bug prevents this route.)
 * Talking to Saltbeef Bob (in the Westin Ranch compound) and giving him a drink of alcohol will get him to tell his yarn. This reveals that a doctor in Shady Sands (NCR) stole his map to Vault 13. After you have the doctor heal you, you can ask him questions. Confronting the doctor gives an option to buy his Velvet Elvis painting for $10,000, which, unbeknownst to the doctor, contains the hidden map.
 * Alternatively, take Tandi's quest to clear/negotiate with the squatters in Vault 15, kill or leave Darion (based on which quest will be done) on Level 3. Opposite from his room is the computer room. One of the computers has information on Vault 13, although it requires a decent Science skill (at least 65). At the vault, you have to either negotiate with or dispose of the inhabitants, which will require the Vault-Tec computer voice module, which can be obtained from Eldridge in New Reno Arms, or Vault 8 (in Vault City).

Or, much simpler, just get to the Control Station ENCLAVE. It can be found in the floor trap room, in the "safe room" all the way to the right. You'll miss a lot of XP by picking this route, but the massacre of deathclaws never happens (or rather, you never learn of them or how they are doing).

Rewards

 * 4000 XP