Well it's hard to tell, GSP is a bigger guy, and even if the fight is at 155 he could rehydrate a lot more as he's pretty muscular. He also consistently trains and is definitely a life long martial artist. I think he could be strong and fast enough to stuff some of Khabib's takedowns.
Conor will always be a dangerous fight because the fight starts standing. And in the first round, Conor is at his most dangerous. Still I don't think it would be more difficulty to beat Conor compared to GSP. The Conor fight lasted as long as it did because Conor was blatantly cheating constantly, even in the round that he won. It probably could have been over in the second round with that kimura if not for the illegal knee and Khabib wanting to show off by getting Dana's attention.