Why would "Canada" test GSP?? It's the UFC's or respective commission's responsibility to test the fighters. Anyhow, he's 5'10 and a ripped 170+. Most big gyms you go to will have a few guys of similar size body fat %. Also, he has a team of people that help him, including a nutritionist. Diaz is just butthurt after getting put into retirement (again). He should be thanking GSP for making him a millionaire.
He was 77.1kgs or roughly 170lbs for the fight. But these guys can cut up to 20lbs of water pre weigh in and then regain weight as its a 24hr weigh in.
As to what he weighed in on the fight night itself id expect it to be more than 170.
Not saying he was using, just saying these guys will mostly be a good 10lbs bigger than their listed weights come fight night.