gsp has never really gassed going 5 rounds at 170, he got abit tired during the fight with diaz but his training team said it was because he was fighting of a flu and felt worn down prior to the fight.
Your also forgetting gsp would and is alot stronger than kahbib, guys like chael who are bigger and past opponents at ww have said it.
That being said even if he gets taken down i think gsp has the strength to get back to his feet. Your also forgetting gsp can just take kahbib down and have success holding him there and using his ground game, could also just submit kabib....even gsp can avoid kahbib take down or stuff most of his attempts and control the distance by making it a kick boxing fight I see no real issues for gsp.
gsp has the strength to get back to his feet and ability to stuff maybe not all but some of the td attempts, so if kahbib's only game plan is to go for td after td then by the the time the 4th round happens who knows how much gas he'll have left in his tank, gsp on the other should still be pretty fresh.
when gsp was in his prime 5 round fights where he was taking guys down all day, staying active, and was fine in terms of cardio....i don't see kahbib winning against gsp in any way or form really.