Some assumptions that I intend to make:
Things in the folio are immutable. Things in the boxed set are influential. Things from the Sorceror's Scroll column are inspirational. Things from the later era don't exist. The year is 576CY. Screw metaplots, this is my Greyhawk.
The setting will be treated as though it is a real place, and that the statistics given are attempts to model that real place in particular game terms. This means that I am free to assume (for instance) that magic works like it does in GURPS without conflict with the AD&D system, since each is simply a model for a "real" magic of the world being described. Ideally, this could mean that any GURPS magic system could be used to model magic, including (but hardly limited to) the basic Magic system or variations thereof, Path/Book, Ritual Path, the various Syntactic systems, Powers, or whatever. However, what I will actually use will be the basic Magic system (for Magic User/Illusionist magic), Divine Favor and True Faith (for Clerical magic and undead turning ability), Tree Magic (for Druidic magic), and Powers (for Psionics). I still have to think about mana levels, but I'm leaning toward
$1 (GURPS) is the same as 1 sp (AD&D), making 1 AD&D gp the same as $20 GURPS. GURPS price lists, especially those in Dungeon Fantasy, will be assumed, unless there is special reason to use the AD&D ones. Because I am nothing if not inconsistent, I'll use the AD&D2E weight of coins at 50 to the pound.
A 1st level character in AD&D will be more or less the same as a 75 point character in GURPS, with every level additional adding about
Racial templates for PC races will be held as close to zero points as possible while maintaining the spirit, and as often as possible the letter, of the AD&D descriptions. This may be difficult. I'm going to try and simplify the races, so that (for instance) "Elf" is a race, and all of the different subtypes are cultural variations of that central species. Or not, we'll see.
Because of the existence of monks in Oerth, there will be Trained by a Master and Cinematic Skills available, probably along with Chi Powers. Of course there will be Martial Arts, and not just for monks, but monks will be the specialists in unarmed Martial Arts and pretty much the only source of Trained by a Master (and therefore of Cinematic Skills). Will they be able to use Chambara rules if they have TBAM?
I feel free to completely restructure religions, since they were not, on the whole, defined in the folio. St. Cuthbert and Pholtus are the only two gods of good humanity that must exist (since they were part of the campaign that led to the folio), though I intend to be guided by the lists in the boxed set. Physical manifestations of the gods will be avatars of a sort. The gods themselves will be unapproachable, ineffable forces at the center of realms in the deep Astral plane. Or something like that. Demons and Demigods, on the other hand, will probably be just powerful (sometimes very powerful) individuals.
Alignment will not be a Cosmic Force. It will be merely descriptive. This is a personal aesthetic decision. As a corollary, the cosmology will not be as concretely defined and systematized as in the Great Wheel Cosmology.
In keeping with Oerth's background, there will be no gunpowder or anything like it. Fireworks will be magic spells, and so on.
I'm not sure how I should handle level-drain. I'm still considering what, exactly, it is supposed to model in AD&D, which should help me to model it in GURPS. My current thinking has it that it is a drain of spirit and self-confidence. To that end, I may just model it as the Terror advantage, which can lower a character's point value due to the Fright Check that results. There's even an explicit option to have it take effect by touch attack (but the powerful undead like Ghosts and Specters may not need the touch limitation; I see in GURPS Fantasy that the Wight there is given a Freezing Touch attack, but I'll probably change that to a Terror Touch). Vampires should be modeled as drinking blood in the normal way, and with Infectious Attack/Dominance, though they should probably get some Terror, too.
If you have any suggestions or thoughts, feel free to share them.