[JMason]

One of the things that attracted me to GURPS was the ability to do cross-genre with the same rules. So a "Thundaar" or "Torg" type of "kitchen skin" setting with magic, lasers, robots, whatever, is a great way to show off lots of things.

You could even convert their exiting D&D characters over and have them brought to this new world (or worlds), if the group would be more comfortable with that.

If you do plan on having them create new characters, I highly recommend either using the "character interview" or something like it then you build the characters... or building them with the players to guide them through. First time GURPS character creation can be intimidating.