I wouldn't be surprised if Nadal ends up with the most GS titles. He is the real GOAT in my opinion. I really wouldn't rule him out of the other grandslams.
Don't know if 5 titles is within him before his knees finally give up but I agree with the latter point. His record against the rest of the top 4 is phenomenal - by far the best.
Federer funnily enough has a losing record to Murray, a horrible record to Nadal and only a slightly positive H2H v Djokovic. And yet despite a poor record against his only actual competitors in his entire career, he's seen as the GOAT because he won a dozen Slams when competing with the likes of Roddick, Hewitt and Philippoussis?
I think Djokovic could well surpass both of their Slam titles. When they all retire there won't be a lot in it in terms of Slams won.