I chose to make him human, because his dragon state is already very anthropomorphic. For this reason I could see the show staff make him human as well, like they did with the diamond dogs. We know Spike becomes a dog when going through the portal, hinting that all those who are dragons in the world of Equestria are dogs in the world of Canterlot High, but as of now it has not been established as a fact.

Anyway, since I like to believe Garble's father is a warrior, I picture his human equivalent as a highly skilled military man who is often deployed.