Hairston has the inside track. The fact that he's not in summer league speaks volumes about what the Spurs brass thinks of him. Granted, he's still extremely raw, but for example McGee is in summer league and he projects as the Wizards starting center. I know they barely qualify as an NBA team, but still. For a guy like Hairston to be held out of summer league by a team is meaningful.
I can't see how he's not on the team. He'd probably have to be astronomically bad in training camp/preseason to even have a chance to not make it. It's just a hunch, but I'd even go as far as to say that if he shows that he make the corner three in particular at a respectable clip (say mid 30's or up) that they'll make room for him in the rotation.
As for Gee, he has more upside as a scorer. He reminds me somewhat of Maggette. The problem is, the Spurs already have slashers, what they need are spot up shooters and defenders.
Ultimately, the Spurs may decide (even after the expected re-signing of Jefferson and the expected signing of Jones or someone of that ilk) that Gee is too intriguing a prospect to cut. They will have the roster space to retain him if they decide that.