Time doesn't wait up for anyone. That is the one thing that should have anyone who's betting GSP really nervous. And even if he's physically fine, still mostly in his prime, etc. it still doesn't mean his timing will be anywhere near what it was. He was just a master at blending his striking and wrestling, and timing was a big part of that.
That doesn't mean you're wrong. Elite guys like him are definitely capable of coming back off long layoffs and performing well. He could do it. But to just assume he will? Man, I think it's a complete unknown.