I heard that players can review the GMs at Gen Con. If that's true, is there a place GMs can check out those reviews?
I haven't heard of any such thing. You can certainly provide feedback to Gen Con though.
The other thing you can do is provide feedback to the event organizer. Once, I GM'd a group and they went to the event organizer and told him how much they had enjoyed my game. That got passed along to me and the event organizer was really impressed that they had taken the time to do that. I was really touched and appreciated the effort that they had made.
If you had a bad GM experience, again, that is something the event organizer would need to know, and they would be able to discuss the issue with the specific GM.
Feedback on GM's can be submitted to events @ gencon.com. Be sure to include date/time and event ID. :)