Izzy's takedown defense is at it's best when he's against the cage, because he has crazy long legs which he can spread super wide making it difficult for guys to lock hands. His balance is also great, which is why he's so good at defending single legs. His takedown defense is not so great in open space though, and we've seen him taken down quite often by well timed double legs (Pereira, Whittaker, Vettori). He also tends to give up his back to get back to his feet when taken down (Vettori/Whittaker fights). GSP's double leg timing was maybe the best we've ever seen in MMA, if he could time a good takedown, I could see him taking Izzy's back and locking in a RNC. Izzy hasn't faced a grappler as sharp or well rounded as GSP.
On the other hand, it is also true that GSP has never faced an elite striker this long and precise. I don't think GSP's style of striking would hold up well against Izzy and there's definitely a chance he could get KO'd.
