I agree actually...
He might only be pretending to want Gabe's help because Satan is out for BOTH their necks, and he himself is not powerful enough to take him on, while Gabe, was most definitely stronger than Satan in LoS, and if he were to regain his former glory, the kind we see in the LoS2 trailer, he would be even more powerful.
After he defeats Satan, Zobek will most likely go to his "OH THE CHARADE IS OVER, OLD FRIEND" routine and reveal his true form and fight Gabe. like, "You have done well... Gabriel... as Promised, I WILL RELEASE YOU FROM YOUR SUFFERING" and try to just outright KILL him.
I feel confident we wont fight Zobek in MoF either- he might not even be present, in this continuity, Zobek- Death- is the mastermind, the big bad of the series. We have yet to fight him, and we will very likely fight him in loS2, as the true final boss. And y'know, im alright with that. It's pretty epic. Im not too sure Cox and MS would really rehash the Satan fight again. I mean, it just wouldnt fit.