as evans said, it comes down to whether or not GSP's able to get through the striking and get it down to the mat. Silva is more than able to catch GSP on the way in with basically anything he fancies.
GSP, in all his bouts, has proven, if not superior striking (pretty much anyone), at least close enough to equivalent (Condit, to whom I'd probably give a good chance in a kickboxing match against GSP)
but I believe Silva's striking and timing is on a much higher level, they probably have the same kind of gap in wrestling, but to GSP's advantage.
the thing is, the bout starts standing.
if GPS steps up his striking (and we're talking hollywood-ish step, like big, big time), he may get a few opportunities to take it to the mat.
I think Silva didn't really mind going on the mat with Sonnen and didn't really respect his game that much, he just wanted to make a good show with his 1st round then get it over with, which he did.
GSP definitely has what it takes to control silva on the ground, I believe that firmly. But what would become his TD offense/timing against someone as unexpectable as Silva, he can punch, kick, elbow, knee, whatever he fancies from virtually any position and situation, and land exactly where he wants. I'm not sure GSP could read his game well enough to accurately time takedowns.
The only plan for GSP is to go for Sonnen's plan, but GSP's smaller and probably less strong too, maybe even too conservative to really take the chance. So there's a good chance to see a frozen GSP here too.
if GSP can take AS down, he can take the fight home with an UD, we may even see silva breaking, GSP will play it safe for sure, this coming from a GSP huge fan.
if he can't, he'll get KO'd or TKO'd in the early rounds.
he should fight hendricks first, just to get some more octagon time (all hendricks has is a puncher's chance, but GSP will be ready for it all day)