Information in this article was kindly provided by Ashley Berg at AutismAdventureTravel.com
The Guest Assistance Card (GAC) is a tool to help people with invisible disabilities, like autism, heart problems, or ADHD, to fully enjoy Walt Disney World. It allows you to get accommodations that help you deal with your disability's effects. For example, if you can't handle waiting out in sunlight, you're provided with a shaded waiting area for attractions with queue lines that would otherwise force you to be out in the sun for an extended time. If you have a visual impairment, you're allowed to sit up front at shows. If your child needs to stay in a stroller, you're allowed to use it as a wheelchair and take it through the auxiliary wheelchair and ECV entrances. If the disability interferes with normal waits, you can use the Fastpass line. However, you don't get front of the line access.
If you need a GAC, go to Guest Services as soon as you arrive at one of the Disney theme parks and explain your special needs. You're not required to bring a letter from your doctor, but feel free to do so if it makes you feel more comfortable. The doctor can simply write your diagnosis on a prescription slip.
The Cast Member will give you a GAC that's good for the entire length of your vacation so you don't have to get a new one each day. You can get it at any of the four Disney parks, and it's good for the other three parks as well. It's good for you (or the person in your party with the disability) and up to five others. The person to whom the GAC was issued has to go on the ride in other for the rest of the party to use it.
The GAC is only needed if you have an invisible disability. Those who use wheelchairs and ECVs can skip the Guest Services stop, since they'll be directed to the appropriate accessible entrances at the rides.