GSP is, to this day, the best rounded and more rigorously prepared WW in history.
His striking is overall very good. He has power in his legs and hands. Flexibility, dynamic range of offensive strikes, and good defense and movement. He is technical, accurate, and compact as well, without leaving himself open to counters and such. He took a rather conservative output approach during his WW reign, but before and after, when he took more risks, he showed he has plenty of power.
He has the best double leg in the business, and overall the best MMA wrestling we have seen, arguably, in the history of the sport, at any weight. His jitz are also top notch. Has great cardio. And he trains extremely cerebrally with some of the very best trainers in the world. Plus he has taken extremely good care of himself, never gets out of shape, and has allowed his body to recover promptly from injuries and such.
With that said, he has lost a lot of weight after contracting colitis, and he's only getting older. He would be severely undersized against someone like Colby or Usman, who can barely make WW.
Still, I think GSP's game is deep enough to overcome these deficiencies. Unlike most others, he has many ways to win, and a very dynamic game to adapt his strategy to every kind of opponent and situation. He also is extremely consistent in his performances, unlike say, Woodley, or Rory, who sometimes look like killers, and others deflated. As GSP himself put it, he performs well when it matters.
Askren vs. Shields or Maia would be awesome to watch though.