Robbie Lawler is one of my favorite fighters of all time but GSP is in a different stratosphere skill wise.
The age is the only thing that MIGHT have made it competitive but GSP just had so many more tools and so many more paths to victory.
Robbie is a perfect "sprawl and brawl" fighter. If he can stuff your TDs and force you into his fight he does well. But his fight is pretty much a boxing match with 4oz gloves.
GSP had better long range weapons with his kicks.
GSP would have a monuental edge if wrestling grappling happened.
But biggest problem for Robbie, GSP was perfectly happy to win while fighting SAFE. Georges is not going to brawl with Robbie. He would stay outside, kick and jab and level change and likely win 50-45 or so.
Robbie had a tendency to load up his shots.
You can get away with that if your opponent is tired too.
But I think GSP would game plan for that and have Robbie exhausted by round 3 or 4 just by feinting and making Robbie miss.