Bob will come back human

Reincarnate implies that Bob, although inhabiting a rock gnome's body, is still human:

The magic fashions a new body for the creature to inhabit...

In this case, Bob is the creature, and he's been allowed through the use of reincarnate to inhabit another body.

This GM has always read spells that bring back the dead as unusable for bringing destroyed undead creatures back from destruction. That is, undead creatures typically just can't be brought back from destruction as the same undead creatures they were before they were destroyed by using spells that bring back from the dead regular, everyday dead creatures. (D&D 3.5e, for example, has the spell revive undead (Spell Compendium 175–6) specifically for bringing an undead creature back from destruction as the same undead creature.)

When both the spells resurrection and true resurrection say, "You can revive… someone who has been turned into an undead creature and then destroyed," this GM has always played that the destroyed undead creature is brought back from the dead as the original and now-not-an-undead creature. (Of course, the soul may refuse to be brought back; see Special Spell Effects on Bringing Back the Dead.)

(As an aside, even other spells like raise dead and reincarnate may be able to bring back from the dead a destroyed undead creature as the original creature. For reasons I've never understood, these other spells say, "A creature who has been turned into an undead creature or killed by a death effect can’t be raised by this spell," yet they omit mention of the possibility of first destroying the undead creature. I don't know whether this is an accidental omission or intentional lessening of their power. Seriously, in this GM's opinion, typically, for every destroyed undead creature there's now a soul in the afterlife, and this is just an oversight, but ask your GM to be sure.)