I kept getting the check engine light and tighten gas cap. Paid dealership (almost $200 including labor) for a new gas cap. Now almost a year later getting same check engine light and tighten gas cap. This time they're saying I need to replace evap vent solenoid for $130. It seems like a lot of the issues with this car are the same. GM should step up and solve them.
Clean off the inside of the gas cap with WD40. After I got all of this dirt out, the "Tighten Gas Cap" error never came back up. Air must have been getting into the system and reducing the vacuum pressure for the gas going to the engine.