What happened to Gabe was the vampire seeing an opportunity to exact his goal for propagation while at the same time humiliating and devastating the poor misguided soul who dared to hunt him.
He could have chosen anybody, but he derives a sick pleasure out of breaking Gabe’s spirit in particular.
Meanwhile,
Aside from being just another punishment for Gabe, what happened to his dad is a rare artistic flourish on the part of the vampire; he reshaped him into a form resembling a common pleasure demon.