This Superman vs Batman style debate is retarded. Gastelum was like 23 when he lost to Magny, had a terrible weight cut, gassed early trying to finish Magny and caught his second wind later in the fight and it was still a controversial split decision. Since then, he's fought guys like Tim Kennedy and Chris Weidman and gave them fucking fits, they struggled greatly to control him on the mat so he's shown vast improvement in the area of critique...and GSP isn't half the size and doesn't have half the strength Kennedy and Weidman have, let's face it. Gastelum has some of the fastest hands at 170 or 185lbs and some of the best combination punching in MMA. Kelvin's young, on fire and still improving, GSP looked like shit against Bisping and was steadily declining even before his retirement you could see it in his performances vs Nick, Condit and Hendricks...he was slower, he was getting tired in those fights, he wasn't his old dynamic self and he hadn't shown any new wrinkles or improvement in his game, only regression.
So, do I think at 185lbs 26 year old Kelvin would beat 36 year old GSP? Youre fucking right I do. I called that Kelvin would get the knockout vs Bisping in under 3 rounds too. I guarantee you Whittaker runs through GSP, if he doesn't knock him out, the decision won't be close...but I'm pretty sure he knocks GSP out.