Looks like you and Lisa got well aquainted. She knows a lot about services for the deaf.
I can help with the GAC. That stands for Guest Assistance Card and you request one from Guest Services at any of the parks. It is not meant to provide immediate access or shorten the wait, just to provide an appropriate place to wait (like out of the sun or in a quieter place) or access without things like stairs. When you get to rides/attractions, you show the card to the CM and it identifies what sorts of assistance you might need. For your child with ADHD that might be waiting in quieter areas or away from crowds. The GAC is usually valid for however long your trip is and is valid at all the parks. You don't need a doctor's letter, but some people with invisible disabilities feel better having one form the doctor or from scholl. If you bring a letter, it should give information in a way that non-medical people can understand it and say simply what would be a problem (like he can't tolerate being close to others and will act out, Would benefit from being able to wait in an area away from crowds).
Using fastpass and looking for shorter lines will also help. We've been at WDW over Easter and by using fp and avoiding things with long lines, we have found we sometimes only used DD's GAC once or twice a day.