Here's why I think GSP loses this fight. They're roughly the same size, so that's not an issue. Now GSP's striking style relies heavily on jabbing to maintain his distance. With Silva, that strategy will be neutralized as he not only has the reach advantage but is well known for his precise counter striking. If you can stop his striking, it gets a lot harder for him to set up takedowns. At that point, he would be forced to shoot from a distance which Silva would see coming and, based on the second Sonnen fight, would be able to defend pretty well.
Basically, I feel that Anderson has more ways to win this fight. Even if GSP does get him to the ground and hold him there, I don't think he can finish him, and the next round always starts on the feet where Silva has the edge. I'm not saying it wont't be competitive. That's just the way I see things going down.