I always wanted to move from D&D to GURPS, but the different system and lack of pre-made material always put me and the players off. Have you tried such transition, how did the players handle it, and was it feasible ?
I switched to GURPS from AD&D 1st back in 1988 and it remained as my primary system since. GURPS works well as a fantasy roleplaying game. It has extensive character customization and a well designed realistic combat system with several levels of detail. For some this is a powerful appeal over the D&D style. Plus it toolkit nature means you can make a highly customized campaign setting that all hangs together ruleswise.
The trick of course is that SJ Games has chosen to focus on GURPS as a toolkit rather than offer ready made out of the box support. But it doesn't mean there is nothing.
First for character creation GURPS distills the myriad choices into what they call a template. This is a package of skills, advantages, disadvantages, and perks that reflect a role, or profession. For example a Mercenary Fighter, Low-life Burglar, Wizard, etc have templates in GURPS Fantasy.
First the GURPS Book.
For a Fantasy referee the following are useful
GURPS Magic - pretty needed unless you spend a lot of time with the core books to roll your own magic system
GURPS Fantasy - the toolkit Fantasy books combination of rules and advice.
GURPS Banestorm - Implements GURPS Fantasy for Yrth, SJ Games in-house setting. You can ignore the setting and just use the rules which comprises a fair amount of the book. It is a fairly typical Fantasy World beyond it's "twist".
GURPS Thaumatology - Give alternate magic rules and in addition options for GURPS Magic.
GURPS Martial Arts - It about ALL type of fighting both with weapons and hand to hand. It includes material on both western and eastern fighting styles including a few off-beat one not covered much.
Then on e23 there is the GURPS Dungeon Fantasy series. Which implements GURPS for a D&D style game of dungeon crawling. It pretty much pure rules but so far light on monsters. In paritcular I recommend Dungeon Fantasy 1 - Characters and Dungeon Fantasy 2 - Dungeons. As for the rest get what interests you. I have them all and there isn't a dud in the bunch but there are of varying usefulness.
With Dungeon Fantasy you have two styles of Fantasy you can readily play with GURPS without much work. The first is the somewhat realistic 125 pt level of Banestorm/Fantasy, the second is the more D&Dish 250 point level of Dungeon Fantasy. Given that it all GURPS elements of both lines are useful to each other.
There are two free supplements to get for GURPS. Natural Encyclopedia which is the closest thing to a monster manual GURPS has and Historical Folks which is a book of templates of historical occupations which is great for making up NPCs.
There is a lot you can get so for the Budget minded my recommendation is
GURPS Core, Magic, Banestorm, Dungeon Fantasy 1, Dungeon Fantasy 2, plus Historical Folks and Natural Encyclopedia.
Then later pick up Fantasy, Thaumatology, Martial Arts (if you want to go into that level of detail), and the remaining Dungeon Fantasy.
Finally there is GURPS Low Tech which is a technology equipment book from the Stone Age to the end of the Middle Ages. It will come out at the end of the year or very early 2011.
I ran my Majestic Wilderlands with GURPS from 1988 to date. I posted a couple of my campaign notes here if you want to see how GURPS can be adapted to a traditional AD&D style world.
I recommend you start with the free GURPS Lite. It's an introduction to GURPS and you can give it to your players so they wouldn't have to spend money to see if they like it.
There are a bunch of free playing aids available at Steve Jackson Games.
My troupe chickened out. When we switched from D&D 3.0, we had quite a few very high level characters (well, ~17th level). We had a brief look at how many points we'd need to simulate that, balked, and decided to "retire" the high-level characters. They're now strictly NPC metagame types.
So instead we decided to start a fresh new campaign, in the same setting, with the "underlings" of those characters.
(Also, I found the change from AD&D 2 to D&D 3 distressing: my psionicist character turned into a monk (it felt like a better fit), and I felt a constant nagging "this conversion's bad: it's not the same character anymore". In retrospect, I think converting this particular character to GURPS would have gone smoother because you're in much more control over your character definition.)
Everyone struggled a bit at first with the rules - hey, it's a whole new game system! - but after a while, for the most part, we just say "roll against stat/skill" like we'd do in D&D -
We still haven't fully adjusted from a storytelling perspective: our combats are fierce, frenetic, and over in seconds. (Remember, GURPS rounds are a second long.) We really need to make more use of Evaluate, Wait manoeuvres and the like.
Other than that, the rules have certain basic principles that are applied evenly throughout the system. Once you have those under your belt, things aren't so bad.
One other point of possible "adjustment conflict" is rate of advancement. Our characters progressed pretty rapidly - 2-3 CP per session, and a session might be half an hour of game time, and occasionally maybe a week or two, to skip over travel time. In a year of game play we'd thus increase ~100 CP. And yet, since we mainly invested in skills, our characters didn't feel like they progressed in the exponentially-more-powerful way we'd felt in D&D. (We regard this as a good thing!)
I think the GURPS Magic spells system is fantastic; I like the way that it's set up as a hierarchical skill tree. If you want to learn to throw ice spells, you first have to learn to create water, etc. This gives your character not just a few cool spells, but a whole arsenal based around a few unifying themes. GURPS Martial Arts is also great for adding flavor and variety to non-magical combat in a medieval setting.
GURPS is a more complicated game than D&D. There is no getting around this. It will have a learning curve for new players, and it will take longer to make characters.
People tend to get entrenched in one game system, and dislike change. If you want to sell your players on a new, harder system, know the rules well, and be able to help them easily without stopping the game for half an hour to look something up. Be able to help them make their characters. During play, don't stop the game for 20 minutes to look up something; if you don't know it, improvise a rule that makes sense, or make a ruling, and look it up after game.
GURPS has a lot of great strong-points that you can sell players on. It has a class-less system, and a level-less system, so there's near-infinite flexibility. You can help them think of ideas for characters that are less effective in a more rigid system: a trickster-cleric, who uses holy spells in conjunction with stealth, or a muay thai(sp?) kick-boxing troll. Sell them on some of the awesome ninja cinematic powers in Martial Arts.
I wouldn't recommend GURPS Vehicles; it's just too darn complicated to create things in it. Improvise your own siege engine stats, or use GURPS Mecha or GURPS Robots.
Also, and this is important: let the players have fun. Err on the side of stretching the system to do what they want, rather than playing chief lawyer, and trying to limit them too heavily for the sake of balance.
I find a lot of players will play anything if you present it to them in the right way. I usually start with a fun, one-off scenario using pre-gens, so my players don't have to agonize over character generation or tactics. It's important that you understand the new rules; but if you don't, just be clear about the fact that you're not sure about a particular rule and wing it - look up the rule after the players have gone home.
After the intro scenario, create PC's for your campaign, together . This will take awhile, since there are so many options in GURPS. Some players may take a LONG time. Plan on one game session just for character generation, and be prepared to finish creating PC's with your slower players later, individually, so the faster players aren't stuck waiting for them.
There aren't many modules available for GURPS; I have no problem using D&D modules and adapting them for whatever system I'm using. It can be a bit of a challenge if the scenario hinges on say, a dragon who is in human form, and dragons can't do that in your game system. Give the dragon a magic amulet or something.
Adapting modules in this way does mean you have to read them carefully, so your changes don't derail the plot. If the transformed dragon in the module is beaten up and robbed, he still needs to be able to transform back into dragon form later.
Finally, give your players an opportunity to adjust their PC's later. Point-based character generation is complex, and novice players tend to spend points on skills and powers that sound cool, but don't really work the way the player thought they would. Letting your players re-allocate those wasted points after a few sessions reduces resentment against the system. Don't let them do this at will, but at pre-planned points in the campaign – say after one month of play, again after the second month, and that's it.
We switched between D&D, GURPS and Rifts when we played. The group just decided to do it. The GM was pretty versed in all 3 systems so those of us who had never played GUPRS or Rifts caught on pretty quick. I'd say it all depends on the GM running it and having some patience while you learn a new system.
At first we struggled a bit with GURPS but after one or two sessions, everyone caught on and it was game on from there. Same with Rifts.
Best of Luck !