I have Grim Tales (and am actively designing a setting to run a long campaign with it), and formerly I had Planescape for AD&D (boxed set and a few supplements).
Normally, Planescape really was a pure D&D setting. Now, in theory, nothing should oppose running with Grim Tales, except maybe where magic is concerned. Here is how magic works in grim Tales:
You must have the Spellcraft skill (any GT class can have it). It is used both to learn a spell, then to cast it (casting check is a Spellcraft check). Then, while a spell known can be cast at will (without any kind of preparation), it nonetheless drains on Constitution: 1d6 points of temporary Con damage per level of the spell. So you cast only 1 or 2 spells per day. Nonetheless, three classes on the six can get the Magical Adept talent, which gives Spell-Burn resistance equal to your Int./Wis./Cha. modifier (as appropriate). Meaning you substract the modifier from each d6 rolled (with the restriction that 1 is alxways a burn, no matter the reduction). Thereafter with each Improved Magical Adept talent, you improve your spell burn resistance by +1 (this is a typo in fact, where it should just improve the casting level; but you may say this is not a typo for a high magic campaign world). As such, a 8th level Smart adventurer with a 18th Int could statistically cast as many spells per day as an 8th level wizard, but also having more fets, more hit-points, and much more skills points.
In a Planescape I would suggest this: only natives from the outer planes could have such GT classes, including defense bonuses and action points, while those from material planes would have to stick with traditional D&D classes. I suppose this could add some flavor, and makes natives from the planes much more exotic.
