Can he beat GSP? Yes.
Standing B.J can definitely beat GSP. He was beating GSP standing in the first fight and I don't see it being any different this time around. GSP has a lot of tools standing, but not enough to handle B.J.
If GSP ends up taking Penn down, I don't see Penn putting GSP in a submission. B.J isn't known for subbing people off his back, just great control. He can definitely work his way back to standing/reverse his position, though. I also don't see GSP doing any real damage on top of B.J. If B.J takes GSP down(wouldn't put money on it), he will be able to control/sub GSP.
The only thing we don't know is how good B.J's cardio is at WW now. If B.J has good cardio at WW and can last, I see B.J being able to sub/strike GSP down for either a finish or decision. If he gets tired, GSP will take Penn down much like the first and hold him with being active enough so the ref does not stand them. I believe cardio, much like all of Penn's fights, is going to be be the deciding facor in this fight.
So.. I actually see B.J having more ways to beat GSP than GSP has to beat B.J.
At the end of the day, you're dealing with two of the best pound for pound fighters in the world, both having tools to beat the other. It is going to be an amazing fight.