I figured I'd start with the hardest part first.  Well, that's not strictly true.  I have most of the other parts worked out already, but magic is a toughie, and some of the other stuff hinges on how it works.

Buffy magic is a very robust system, but it doesn't fit the Gargoyles flavor.  Buffy Sorcerers have an entirely diferent set of problems than Gargoyles ones.  The high probability of spells going awry doesn't really fit, and the progressive -2 per turn seems a little off, too.

All we know about magic comes from Demona, the Magus, the Archmage, and various faeries.  And really, there's not a lot.  Normal humans hardly ever cast spells (compared to the amount of time they spend doing other stuff).  Faeries, on the other hand, sling magic around all the time, although it does seem to wear them out after a while.

For humans and gargoyles, the rule seems to be that it's very hard to cast spells spontaneously.  The Magus only ever does it once, and he dies from it.  I think the Archemage throws around some nasty spells, but it's been a while since I've watched it.  I'm going to go with the assumption that for the most part, magic in Gargoyles is through rituals, or through using magical devices.  For instance, it was likely a property of Titania's Mirror that Demona could use a simple ritual to summon Puck - rather than a "spell" of Demona's.

EDIT:  Something else I've thought of

One thing I might change, although I'd do so with some trepidation, is to make spell formulas a little more flexible.  Spell R&D is pretty hard.  If your books don't have a spell for turning people into statues, you're probably never going to be able to turn people into statues.  But if your books DO have a spell for turning people into statues, you can probably turn one person into a statue, or a dozen, or a million if you're good enough to marshal the magical energies.  And you can do so under different circumstances, like "the next person I touch" or "everyone in the sound of my voice."

What this means is that the spells you know are listed with their minimal effect and starting PL, and you can pick and choose modifiers.  Make the spell work longer or faster or bigger, and the PL goes up.  Make it take rare ingredients or year-long rituals, and it goes down.

That's a scary level of freedom, though.  I've already learned that WitchCraft style Invocations are frighteningly poweful, and those just let a magician do one kind of effect.  So I'll have to think about it.  Sometimes play balance has to win over simulation of the source material.

So anyway, the Sorcery quality is really, really rare.  Human magicians learn rote spells and need either elaborate rituals or a magical conduit to work their mojo.  A magical conduit is an Enchanted Item that - just to keep things simple - provides the holder with the ability to Quick Cast spells up to its Power Level.  It doesn't provide the bonus points.  Instead, that's going to be a Quality called - because I like GURPS - Magery.  Levels of Magery up to 5 add to your Occultism skill to cast spells.  Since I'm already dinging you at roughly 2 points per level for Quick Casting, I'll just make that Quality a 1-point per level Quality.  And if you want Telekinesis, you have to pay for it the old-fashioned way.

I think I'll leave Sorcery in, but make the prerequisite for learning it be something like Occultism greater than 10.  And I might turn it into a sort of "Wild Card Skill" so it advances a lot more slowly.

On the other hand, Gargoyles magicians don't seem to have too much trouble with spells going awry.  A spell can fail if you're not talented enough to cast it, and what it's supposed to do might not actually be what you think it does, but it's not going to do something radically different.  Magic seems very legalistic.  A spell to make you as enduring as the mountain stones may just turn you to stone.

(Although I have a private theory that if Xanatos had immersed himself in the cauldron, it would have made him immortal - he would have shed his stone skin like a Gargoyle.  I think Puck didn't want Xanatos to be immortal because then  he'd be stuck as Owen for all eternity, so he did something to scare Xanatos off.  Greg Wiesman doesn't actually agree with me, but what does he know?)

Faeries have it different.  They seem to be able to just whip out with spells that do just about anything at the drop of a hat.  They also DO seem to suffer from some degradation of their powers.  Puck says he'll need a long nap after The Mirror.

Ergo, Faeries DO have Sorcery, which costs 5 points per level and lets them Quick Cast spells.  I'll keep the Quality at 5 points per level even though it doesn't include telekinesis because it will include the ability to improvise spells.  Faeries just whip out with the magic whenever they want.

I'm also making one other major change - Magic can kill you.  If you throw a Quick Cast spell of a higher PL than your Sorcery or available conduit, you take physical damage from it.  I think I'll make it five times the PL of the spell.  That'll REALLY limit what magicians can do without their tools.

Faeries can also have other powers.  In fact, not all faeries or demifey need to have Sorcery.  They might rely on innate powers that are more reliable, if less versatile.  For those, the Angel supernatural Qualities will do most of the job.

So then there's Enchanting and Superscience.

Magic items are really rare in the Gargoyles-verse, and most of the ones I can think of are actually Faerie creations: the Eye of Odin, Titania's Mirror, the Phoenix Gate, and probably Excalibur.  The Grimorium is something of an unknown, as is the Gae Bolga.  I think I might limit Enchanting either to just level 1 for humans, or entirely to Faeries.  Or maybe not.  There's a dearth of evidence on the subject.

Superscience, on the other hand, should be easier than it is in the Buffyverse.  Robots, blasters, floating factories, and whatnot are all common enough not to raise eyebrows, even if they're not ubiquitous.  So Superscience works as written - you have to have a high Science skill to have it, and it costs 5 points per level.  The only difference is that Superscience gadgets no longer require a special ingredient.  Instead, rare ingredients can lower the effective Power Level like they do for Sorcery spells.

And I think that covers everything magic-related.  We're going to discuss a slightly different way to approach the Enchanted Item Quality a little later on.  It works okay for some devices, but is really bad for things like exo-armor.

If I get my hands on All Tomorrow's Zombies before I write the next segment, I'll see what Eden did for battle suits there.  I think they went with a variation of the Robot Quality, which probably works pretty well.