Here is my reasoning for why GSP would win:
In MMA, 80% of fights end on the ground because it is so easy to take an opponent down, so the fighter who has the best ground game is the favorite. GSP is much better wrestler than Anderson, so he will take Anderson down and either get him on his back and ground-and-pound him or submit him. Anderson has no submission defenses to speak of.
Can Anderson KO GSP before GSP takes him to the floor? Sure, but it's unlikely. GSP is like a mini Nogueira in his ability to eat punches, and Anderson lacks power. Anderson is probably the most agile and technical striker in all of MMA - that flying knee was the sickest thing I've ever seen in MMA -, but nothing suggests he has the stopping power to lend GSP on his back. Every time that Anderson starts delivering his bombs, GSP will clinch with him until he finally takes him down. The judge will separate and GSP will clinch again. It is impossible to avoid being taken down ad infinitum. GSP will keep his distance and avoid Anderson's legs and then rush towards him and clinch. He will clinch, clinch, clinch until he takes him down. Once on the ground, GSP will grind out a decision via ground-and-pound or he will catch one of Anderson's limbs and finish it. If GSP can avoid Anderson's legs and clinch, he will take him down and win either by points or submission.
SUCKMYMUSCLE