You don't need a doctor's letter. Some people (especially with things that are not visible) feel better having one.
Here's king of a summary about GACs (Guest Assistance Cards):
Go to Guest Services in any of the parks and talk to the CM there about problems and needs. The diagnosis is not really that important because people with the same diagnosis can have very different needs. The GAC is not given based on diagnosis at all (for example, there is not a "list" of appropriate diagnosis for a GAC); the GAC is given based on needs and the accomidations that meet those needs.
The GAC is usually given for up to 6 people (5 plus the person with a disability) and is valid at all the parks for your length of stay (you don't need to get a new one at each theme park).
A few other things
- the GAC is not meant to be a pass that gives immediate access. In fact, about 6 years ago, they renamed it to Card because when it was called a Pass, people thought it mean front of the line access. It says right on the card that it will not shorten or eliminate waits in line.
- it is meant as a tool to tell the CMs what the person's needs are so that they can provide appropriate accomidation for the person it was issued to. Some examples of accomidations might be a quieter place to wait, a place out of the sun (for those times when the line is in the sun for a prolonged period of time).
- because what is stamped on the GAC is based on needs, not all GACs say the same thing.
- think of the GAC like an insurance card, not necessarily needed or used all the time, but there for when you need it.
- even on the same attraction, the GAC is not always handled the same each time. Exactly what happens depends on how busy it is, how many other people with special needs are there at the time and staffing. Some times you may be sent thru the regular standby line, occassionally the fastpass line; sometimes the person with the GAC and a member of their party will be given an alternate place to wait while the rest of the party goes thru the standby line - and then meet up with them when they get to the front. Sometimes you might be given a slip and told you can come back at the time written on the slip (usually equal to the standby time); very occassionally, you might be taken right in. It depends on what they call "attraction considerations" (which is basically the things I listed in the second sentence).
- Fastpass is a good way to avoid waits in line and will be very helpful to you. . You don't all have to be present to get a fastpass, you can send one member of your party ahead with all the park passes to get fastpasses. When you report back to the ride at your fastpass return time, your wait will be 15 minutes or less.
My youngest DD has ADD and we have been to WDW with my nephew with ADHD (Attention Deficit Hyperactivity Disorder). We do get a GAC for our DD because she has other issues with having to wait in crowded conditions. But, even during busy time like Easter/Spring break, we try to use the GAC sparingly. The one thing we have found is that the more the child wants to go on the attraction, the more pleasant the wait is. Our worst waits on the trip with my nephew were the Country Bears and Splash Mountain, neither of which my nephew wanted to go on. But there were some other attractions which he waited patiently for a much longer time.
Also, even during busy times, if you can get to the parks at opening time, you will have much shorter waits. DH and I have been at MK in July (normally a very busy time) and were able to go on Peter Pan, Pooh (twice), Snow White (twice), Haunted Mansion and Pirates without waiting for more than a few minutes by going early. By 11 or so, the parks get quite a bit busier.
The Unofficial Guide has a lot of information re: touring in ways that shorten the wait and avoid crowds.