At the moment in my opinion, Gavin does indeed control Bitcoin Project to some extent. He is widely recognised as the main developer behind it and could probably push his authority on a lot of subjects if he so desired. However, I am under the impression that he would not perform such actions - they would undermine the credibility of Bitcoin and be bad for everyone.
He does appear to have a lot of influence over a lot of aspects of the Standard Client, the Protocol and overall behaviour exhibited by Bitcoin. He will most likely have the final say for example on the subject of block size limit. However, if he would change any more major aspect of Bitcoin that would undermine a lot of other people's work, say by changing the mining algorithm hard coded into the ASICs, it wouldn't be long before he would lose his position of authority and possibly create a hard fork.
So to sum up, at the moment Gavin is de facto the person who controls Bitcoin on its most basic level, and he will most likely hold that position of authority only as long as he remains a "benevolent tyrant" so to speak.