The GURPS magic system is extremely versatile (especially when considering options given in GURPS Magic, GURPS Thaumatology, GURPS Fantasy and GURPS Dungeon Fantasy). It is also very different from the “normal” D&D magic rules. This means two things: 1) The two systems are different enough that the D&D system really can’t be duplicated with GURPS rules. On the other hand, this means that 2) we can really customize a magic system for a particular class. In my opinion, one of the weaknesses of the d20 magic system is the lack of distinct mechanics for magic that is conceptually very different (e.g. in d20, a Sorcerer is “simply” a Wizard who doesn’t have to memorize spells). Since the GURPS magic system needs to be customized anyway, why not customize these different concepts.
The fundamental mechanics of the base GURPS magic system will be used: spell skill check as “caster checks” to determine if a spell is successfully cast, DX-based skill to target missile spells, fatigue point costs to cast, range and area effects, and so on. The primary difference between classes lies in which spells are available and how they are obtained: