I won't comment on the grappling aspect since that's not my main area of knowledge, but the striking part is pretty interesting. In striking, Nick Diaz is a specialist, and by that I mean that when it's strictly boxing he's damn good at it and pretty much unmatched. If you trade punches with him you will not win.
However, boxing is only one part of striking, there's also kicks, knees, elbows, and most importantly, footwork to setup all the above. The last one is Diaz's main weakness along with kicks, and that is where GSP can take him apart if he's smart.
Diaz uses a very side-on and fairly upright boxing stance, this extends his reach and helps setup his lead strikes, but it also leaves him vulnerable to low kicks and takedowns. Diaz doesn't have fast footwork and the stance slows him down further when he's trying to turn to his right to chase or cut off an opponent. Note how Condit was able to spin away to Diaz' right and reset towards the center of the cage nearly every time he got backed into the fence. With a wider stance Diaz can use a lead hook to cut off an opponent who's stepping towards his right without moving his feet to reset his angle, this gives the opponent little time to get away. With his narrow side-on stance, Diaz has to reset his feet before he can throw a strike, and that's what gave Condit the time to get away from the fence.
There's no question that GSP has much faster footwork, and he's also shown great ring awareness in that he always controls the distance and stays away from the fence. He should have little problem circling to Diaz' right where he's safe since Diaz will have to punch across his own body to hit him. As I've learned from over 20 years of various striking based martial arts, you have no power or reach when throwing strikes across your body against someone who has the angle on you. You need to reset your feet first, and while you're doing that your opponent is taking free shots on you.
If GSP and his coaches are smart they're going to exploit this for all it's worth. Start with the left low kick to right cross left hook combo and exit to Diaz' right. Once Diaz starts picking up on that, start with a superman punch or right straight down the pipe and exit with a left leg kick. The footwork is very similar to the first combo. Next combo, right kick to the body, left hook to the body or head, exit. Every combo starts with very similar footwork and exits to Diaz' right. There's nothing fancy and they all use strikes that GSP is good at and used to good effect in previous fights.
GSP can't match Diaz in boxing, if he tries he'll end up like the rest of Nick's victims. What he can do is use his superior speed, footwork, and kicks to take Diaz apart while staying out of the killzone. If he's smart he'll do that and pick up an easy win. If he gets sucked into Diaz' game, all bets are off.