Kelvin is an absolute dog, and a very difficult fight for anyone. He gambles high and is very forward. He caught Bisping clean and got the KO.
GSP is a very good kickboxer, who doesn't take crazy chances. He would not open himself up idiotically to someone like Kelvin, and he would try to get him down. Could he resist GSP's takedown? Maybe, it's hard to say. GSP is older and bigger, stronger probably, but not nearly as fast and with clearly slower reflexes than in his prime about 7 years ago. Age is real. But he is a crafty fighter, with improved boxing, with power of his own, excellent kicks, and top tier jitz.
It is amazing how people continue to question GSP. Anyone who has actually witnessed what the man has done cannot with a morsel of intelligence doubt he is a superlative fighter and athlete.