I think Law is too smart and tricky for Luffy. Luffy's speed might be a problem. I think the chances are 50%-50% for those two. Law could beat Luffy by getting his heart, using a smart plan. It depends if Luffy is smart enough and fast enough to avoid Law and be able to attack at the same time.
Actually no. He is not as stupid as he is in everyday situations but not by any chance can he be categorized as smart. In most of his fights, he gets caught up to the same techniques and uses the same (failed) methods until he realizes he needs to do something else. His good stamina and the fact that he can take a lot of damage is what saves him from being seriously wounded. With Enel he had a natural advantage. Look at his fights with Caesar, he got caught up to the same thing more than twice. About his gears, it only natural to understand his own body and ability better than anyone else, I don't see why he has to be genius to develop that.