BJ Penn is fat and needs to shut up!
GSP is not using riods as Claeren said anyone could look like that with enough work and eating right.
He has people that wake him up, push him hard every day and make/plan meals for him every few hours. It is a lot easier to wake up at 6am when you know you are going to be the one who is just yelling at the other guy to do push ups, so finding motivation for him is easy because he pays someone to do it.
Look at how small his arms and cheat are. If he did roids and worked out like he did he would be much bigger. He looks more lean than muscular to me.