Wulf and jez have pretty much laid it out, but just because it's a chance to pimp, hey, here I am!
You should find HP a great deal of value as an add-on to GT. While I was definitely directly inspired by Grim Tales, these rules are by no means the same set. As Wulf says, they're an expansion on what GT had -- and they're also much more carefully aligned with d20 Modern. They should fit just fine into either game type (not to mention straight-up D&D as well).
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Hot Pursuit rules don't make any assumptions as to available classes, feats, technology or magic levels. They assume that the more common d20 skills are available (Drive or Ride, Hide, Spot, Bluff etc), that saving rolls and ability checks work in a more or less standard fashion, and they include suggestions on adjusting DCs depending on the type of game you're running (the level of swashbuckliness, basically).
The basic differences are:
Spycraft depends on two parties making secret choices about their maneuvers, then revealing those choices and comparing them to get a result. Sort of like the way psionic combat used to work -- the attacker picked their attack mode in secret, the defender picked their defense mode in secret and you cross-referenced those to get a bonus or penalty or whatever. It makes chases feel very different from the rest of the d20 world -- which isn't good or bad, but that's what you get.
Grim Tales uses a notion of "Reaction Rolls" -- where instead of comparing one maneuver against another, players go in order of initiative and choose maneuvers. Some maneuvers require a Reaction Roll where your opponents make an opposed check to resist your efforts. It makes chases feel more like standard d20 combat does.
Hot Pursuit takes the Grim Tales system and expands it out to create a full-blown, standalone chase system that gives chases all the detail and color that combats have in "standard" d20. My intention was to make it as easy to integrate chase scenes into my many and varied games as it already was to integrate combat scenes.
I'd love to know what you think! Don't be shy with your questions!