Actually neither of us is correct. There is no quote that says that Gauldoth is a vampire, nor is there any quote that says that he is a zombie. There is no canonical statement as to what he is. He could be zombie, liche or vampire.
By the way, how exactly did you extract those text files?
Anyway to the reason why I think that Gauldoth's undead half is a vampire.
I slapped him with all the strength in the undead half of my body. The blow might have killed a living man.
As an answer, I crushed the holy man's skull against the nearest wall, and then I used my magic to open the sarcophagus silently. Korbert's dry, rotten body rested within as if he were sleeping. Clutched under his arms were his sword and shield. I didn't care for the sword.
Gauldoth's undead half is extremely strong, so much that without using a weapon he can basically crush bone. Zombies cannot be that strong, else they would be a far higher level creature than they actually are, hence I feel that only a vampire is sufficiently powerful to do justice to that kind of power.
We know that temporarily we can create vampires using spells and also through necromancy in Heroes IV.
For another reason I think he is half-vampire, look at the portrait.
Compare it with the Vampire.
Or the zombie.
The vampire has red eyes, the zombie doesn't. Gauldoth Half-Dead is a vampire because he has the cleaner smoother look of the vampire with the red eyes, rather than the mangled zombie look.
Although he doesn't have gray skin.