Okay Deckard being DEFINITELY a Replicant, prior to having Replicant Bladerunners, finally, FINALLY explains why Gav so hates him in the original movie.

Presumably Deckard is the first Replicant assigned to being a Bladerunner, and Gav's assigned to tail him, and kill him if he goes rogue. So instead of handling a big case that could make his career, he's keeping tabs on the detective assigned to handle that big case.