GURPS allows you to play in any genre. Many people prefer GURPS rules to ones published with other games, but wish to use the background provided by other games. Conversion systems allow you to use the background from other game systems while using GURPS rules. Adaptations describe a world in GURPS terms.
Below are sources of info converting various game systems into GURPS.
Simply put, not every depiction of the future is going to mesh perfectly with the Tech Level system presented in GURPS. However, with a little fudging, one Tech Level can usually be found that will cover the majority of a given fictional setting. What still doesn't fit can be considered a "wild-card" technology that the setting just happens to be much better (or worse) at than would be expected. For instance, the setting in the movie 12 Monkeys might be considered vanilla TL 7, with the exception of prototype time travel (TL 15 or so).
That having been said . . .
Star Wars: TL 12 (Hyperdrive on an X-Wing, Force Fields on everything fighter-size and up), with TL 13 in the more advanced areas. TL 13 Contragravity.
Star Trek: TL 15 (Disintegrators, Tachyon Communicators, etc.), with TL 16 Teleport Projectors and TL 9 AI (Data and Lore are just about the only ones).
"Guns, Guns, Guns, 3rd Edition" (a.k.a. 3G3), by Blacksburg Tactical Research Corp. (BTRC), is a universal guns/weapons design and translation system that allows you to create weapons for any RPG and transfer them between RPGs. GURPS is one of the systems specifically supported. Conversions also exist for Timelords, CORPS, CP220.127.116.11, Megatraveller, Torg and others. Highly recommended if you like rolling your own weapons for GURPS or other RPGs. The conversions for GURPS are usually off by 5% or so for range and damage.
BTRC has also published "More Guns," a compendium of firearms which includes stats for the weapons in GURPS terms.
GURPS Vehicles, 2nd Edition contains a weapon design system that is quite useful for converting weapons into GURPS terms, although some number-crunching is needed to translate real-life stats (caliber, weight, et cetera) into game terms.
Some suggestions on converting AD&D to GURPS:
Concentrate on Concepts: Don't try direct conversions. The best thing to do is to convert concepts. To convert a character, write down a description of your character in plain language, with no gaming terms. Include his personality traits, likes, dislikes, fears, and so forth. Then, take the description and write it up in GURPS stats. You'll get the feel of the character, which is a lot more important than just transferring stats over.
Hit Points: AD&D's hit point system reflects increased skill luck, and knowledge with combat tactics. GURPS represents the "skill" aspect of hit points by increased weapon skill - the higher your skill, the higher your active defenses.
DnD 3rd Edition, with its new Attribute scale, and use of Feats and skills, is much easier to convert to GURPS than AD&D. Attributes will probably need to be lowered a bit. Many of the feats can be translated to GURPS advantages; for example, bonus to saving throws could be represented as b Will, Weapon Focus as a bonus to skill, Weapon Specialization as Weapon Master (one weapon), etc. Most DnD skills corresponds directly to one or more GURPS skills.
As said above, the high hit points of DnD can represent a broad array of abilities in GURPS, like high HT, Extra Hit Points, Toughness, good defense rolls, High Pain Treshold, Recovery, Hard To Kill, etc. Don't forget that in DnD, your character dies when reaching 0 HP. In GURPS, this is not the case; someone with a high HT could survive way into negative hit points.
The Roleplayer #19 (April 1990) article "Converting a Campaign to GURPS" by Rich Ostorero has the outlines of a Battletech-to-GURPS conversion system.
"Cthulhu Lives!" by David Ellis Dickerson, Roleplayer #22 (November 1990), has conversion rules for Call of Cthulhu.
Also, SJ Games has published GURPS Cthulhupunk under a one-time license from Chaosium. Cthulhupunk is a near-future/horror adventure RPG which uses Chaosium's Call of Cthulhu material and GURPS Cyberworld. It contains a fair number of conversion rules as well.
Source: Sean Barrett
None are currently available. Alan Shock and Jim Duncan developed a draft set of these conversion rules. Currently no work is being done on these rules. Jim Duncan would welcome assistance on this project.
"A Whole New World: GURPS on Harn" by Michael Cule, Roleplayer #26 (October 1991), has rules for running a campaign set in Harn.
"Super System Switching" by David Ellis Dickerson, Roleplayer #21 (August 1990), has rules for converting Hero system characters to GURPS. The article concentrates on Champions to GURPS Supers conversions. It's based on pre-4th edition Champions, so some adjustments may be necessary.
Also, Fantasy Hero has rules for converting GURPS characters to FH, so you can use the inverse.
The best conversion is of course the GURPS In Nomine book, a complete conversion of the In Nomine setting to GURPS. S. John Ross wrote an online In Nomine to GURPS conversion system, with contributions from Moriah, Hunter Johnson, and Dr. Kromm. Comments should be addressed to Hunter Johnson.
This RPG and multi-genre game setting contains conversion notes for GURPS. Much of the setting seems ideally suited for running a GURPS multi-genre game.
Source: Jim Duncan
The Force can be handled as a combination of Cinematic Force-Swordsmanship (p. MA114), psionics, and maybe the Weapon Master advantage (p. CI32). Luck and Destiny might also be appropriate.
Source: Dr. Kromm
Steve Jackson Games formerly held the license to produce GURPS material using this background: one source book and one other book for each game. GURPS Vampire: the Masquerade, Werewolf: the Apocalypse, Vampire Companion and Mage: The Ascension were all published but are now out of print. These books are set in White Wolf's "World of Darkness" setting, and contain rules for converting between GURPS and the Storyteller system. No further adaptations will be published.
German and French translations of GURPS Vampire have been released by Pegasus Games and Asmodee, respectively.
In addition, several unofficial WoD conversions can be found at the GURPSnet-L World of Darkness Setting.
A complete line of GURPS books cover the Traveller settings extensively. More information is available in the GURPS Traveller section of this FAQ.
Check the official list of GURPS Traveller books for current and future releases in this series.
Aaron Sherman has written a GURPS conversion of Robert Jordan's Wheel of Time series, but it is no longer available online.