[@Remipa Awesome]
Plot, mainly. Gabriel won't mindwipe Hank because it wouldn't be fun for you, which is a bad thing. IC, he won't do it because he likes Mithias, and as such is prepared to help him with this issue.

That's not to say Gabriel isn't prepared to screw with Hank's head though. He might rig the memories to explosively return the moment Hank tastes peanut butter, or something like that.