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 concept of the device as well as its design was done by Doctor Stanislaus Braun, a brilliant if egotistical Vault researcher.

Appearance
The kit looks like a small, silver briefcase emblazoned with the letters G.E.C.K.; and, according to Vault-Tec advertisements, it contains all the seeds, fertilizer, and other equipment - including a cold fusion power generator and a basic replicator - necessary to start a new settlement after emerging from shelter. Each Vault was presumably issued with two GECKs, except for Vaults 8, 13 and Vault 101; 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 as backups.

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.

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.

Actual contents
The GECK 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.

Fallout 2 MacGuffin
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.

Fallout 3 MacGuffin
When the Vault 101 Dweller locates his father in Stanislaus Braun's Vault 112, he learns that in order to make Project Purity work properly, he requires a GECK. After his father's demise, the Dweller makes his way to Vault 87, a horrendously radiated location, where he acquires a single unit.

Not consistent with previous games is the effect the GECK has - when activated, it kills the player, but a large area of fertile landscape erupts around him. Obviously, this is not even comparable to what the GECK originally was - a survival kit that (ironically) wasn't necessarily allowing survival with its outdated contents.

A way to reconcile this with other games is that unlike most GECKs, the one found in Vault 87 was actually built entirely according to Braun's specifications and thus a true terraforming device. Of course, the resources needed to construct such a powerful device would be immense, and with the American industry strained by the war with China, Vault-Tec would have to design a cheaper, more cost efficent device to issue to Vaults - the GECKs we find in Fallout 2.

There is, perhaps, a simpler explanation: that the GECKs were always terraforming devices (one is used to make Vault City and New Arroyo, after all), and that the description of the GECK as nothing special found in the Fallout Bible (and only the Fallout Bible) was simply dismissed.

Another possible explanation is due to the fact that the GECK shown in Fallout 3 was stored and used in a high-radiation environment unlike previous GECKs which underwent neither, possibly mutating it's organic contents, a possible nod to the plot of the would-be Fallout Tactics 2.

And an even more logical explanation is that this a JOKE, and not canon in any way possible. Unless you consider all the jokes in the game canon.

Appearances in games
The GECK first appeared in the Fallout manual and later it became an important part of the Fallout 2 plot. GECKs are also mentioned early in Fallout 3 and later serve as a MacGuffin again.

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.