Another misconception is that you somehow have to "prove" disability in order to get a GAC. The Americans with Disabilities Act (ADA) makes it illegal for Disney (or anyone) to require such proof. Guests need to explain what accommodations they require, not what their disability or diagnosis is. Don't bring a doctor's note- they won't want to see it.
I've never used a GAC myself, but I know of a few families who would never be able to experience the magic of WDW without one. Do you have any experience using a GAC? What are your thoughts on the process?