"No, not the Fighting Fantasy Steve Jackson, the GURPS Steve Jackson". I hear it quite a lot; it's an easy mistake to make, and an assumption I held myself for years.
GURPS (General Universal Role Playing System) publish a whole range of interesting background variants for the entertainment of the roleplaying public. It has to be said that Pratchett's fantasy Discworld is hugely popular and etched into the public consciousness in a way that allows instant comprehension of the setting by experienced gamer and layman alike.
But does this work? GURPS is not an especially user-friendly system, for all its cross-genre potential, it is decidedly clunky in many areas, not least character generation. I wouldn't advise trying this until you've got your head around tabletop roleplaying, maybe something accessible like Dungeons & Dragons or Warhammer Fantasy Roleplay. The Discworld games always suffer in practice because Terry Pratchett isn't actually sitting at the table being innovative and witty. Games tend to trail off into a recital of half-remembered plotlines, non-player characters, settings and events. This gets more confusing these days, when the volume of Pratchett's excellent novels lends itself to a reasonably strict chronology.
So, it's an interesting read for a Pratchett fan who's a gamer. But don't expect anything new here, and a Pratchett obsessive will find this repetitive. There are better games than GURPS, and even within GURPS you'd be better considering their Illuminati variant or any one of many other comedy or more user-friendly game settings.