Most of the companies I know only let their senior guides who have been with them a while guide on Gore. So your odds are low unless you work for a company that runs the Ark and Gore. Check out Breckenridge Whitewater Rafting, Timberline Tours and Lakota in Vail, Clear Creek Rafting near Idaho Springs, and Arkansas Valley Adventures and Dvorak's on the Ark. Those are the companies I know of that run Gore and the Ark - there might be a few more.
Also worth mentioning - I don't think there is much demand/work for Gore trips in September - it is nothing like the Gauley in terms of the number of people who raft it.