He's a threat as long as the fight is standing, that left hand of his is no joke. I'm pretty sure GSP will take him down without much trouble tho. He's taken down accomplished wrestlers before. I'm not that impressed with Hendricks wrestling thb. Not to say that it's by any means bad but he hasn't shown anything to prove that its on another level either. I dont think his mma-wrestling is any better than Kos, Hughes, Sherk, Fitch or any of the other wrestlers that GSP beat already.
Also his topgame looked pretty bad & I can just imagine that his bottomgame is pretty horrible, as strong wrestlers seldom develope a good guard.
My prediction is GSP will be a bit scared of Johnnys power, he won't take much risk standing with him, but he might jab him around a bit. Then he gets one takedown per round and keeps Hendricks down pretty easy.