GECK

The Garden of Eden Creation Kit or GECK was a device created by Future-Tec, a division of Vault-Tec, which was meant to revitalize areas of the wasteland.

The kit looks like a smallish, silver briefcase emblazoned with the letters G.E.C.K.; and, according to Vault-Tec advertisements, it contains all the seeds, fertilizer, and gizmos - including a cold fusion power generator and a basic replicator - to start a brand new city. Each Vault was presumably issued with two GECKs, except for Vaults 8 and 13; Vault 8's extra GECK got swapped (whether by accident or some nefarious scheme, this is not known) with the Vault 13 surplus water chips that were ordered when the one working water chip broke.

The GECK, however, isn't really a replicator. It contains a fertilizer system filled with a variety of food seeds, soil supplements, and chemicals that could turn arid wasteland (and possibly selected sections of the moon's surface pre-conditioned to accept the GECK) into fertile soil for farming. The GECK is intended to be "disassembled" over the course of its use to help build communities (for example, the cold fusion power source is intended to be used for main city power production), and so on. For anything else people needed, they could simply consult the How To Books/Library of Congress/Encyclopedias in the GECK holodisk library for more knowledge. The pen flashlight was just a bonus.

The GECK also contained some basic force field schematics as well as info on how to make adobe-type buildings from the landscape (or it contained chemicals that could create "sand-crete" walls).

As for clothing, the GECK contained codes that allowed the Vault to create more varieties of jumpsuits (and weatherproof gear) from their dispensers, which they could do anyway before the GECK. It's possible the GECK contained other codes that could unlock more functionality within the Vault computers that weren't initially available because they would jeopardize the survival of the Vault if they were used or scavenged (or else they would interfere with the Grand Experiment).

The GECK could also tell the Vault inhabitants how to disassemble sections of their Vault (or take extraneous systems from the Vault) to create new homes and defensive structures on the surface.

The "just add water" comment/joke for the GECK in the advertisement refers to the fact that part of the GECK's operations require that the Vault Dwellers use water from their water purification system in the Vault to help with the agriculture, irrigation, and possibly the cold fusion as well. It wasn't meant literally. If you want it to be, that's cool, too. Go for it.

To close, the "basic replicator" mentioned in the Vault Dweller's Survival Guide is nothing more than a selection of seeds and fertilizers. The fact that it can "build basic items" is intended to mean that you can use it to help break down sections of the Vault into items usable in a community, as well as provide new codes for the machines in the Vault to create new items from the dispensers and computers.

The seeds and soil supplements date themselves rather quickly, but the government subcommittees sponsoring the research and the GECK contractors (Future-Tec) weren't really concerned about that. They were "relatively certain" the seeds would be viable in a post-nuclear environment. They had done "thorough tests," and "all conclusions point to this as being the best option." The GECKs are a miracle... a miracle that they work at all.

The GECK builders had no idea what the post-nuclear world would be like, and they had no real way to anticipate it, despite their "thorough tests". It's doubtful they gave it much thought, to be honest, considering how badly organized the Safehouse project alone was, not to mention the experimental nature of the Vaults themselves. Still, it seems as if the seeds present in the GECK were viable for Vault 8. The GECK designers assumed that the Vault Dwellers would know how to read and how to operate various technologies present in the Vault - they didn't plan for tribals or other contingencies. They also didn't plan on the FEV getting released, or the fact the Vault Dwellers might be attacked by giant mutated scorpions or rats, either.

When the Vault Dweller traveled north and founded Arroyo, he probably took one of the Vault Dweller Survival Guides with him. After he left, the Arroyo tribals assumed that the "Holy GECK" advertised in the Guide was a sacred, magical item which can change the wasteland into fertile land again. The tribals were invoking the GECK as a panacea for all their problems and saw it as a miracle device. But while the item is useful, it's not the miracle maker they considered it to be.

Still, while the GECK simply isn't the holy cure-all miracle device the inhabitants of Arroyo intended it to be, in the right hands it's a very useful piece of Pre-War tech that can help establish a viable community. After the destruction of the Enclave, the dwellers of Vault 13 rescued from the Oil Rig used it to create New Arroyo.

Appearances in games
The GECK first appeared in the Fallout manual and later it became an important part of the Fallout 2 plot.

Behind the scenes
According to Chris Taylor, the GECK was created by Jason Anderson and Leonard Boyarsky for the F1 manual - it wasn't intended to be used in Fallout 2. But as far as a McGuffin goes, it was there when it was needed.