Lisa is right, you don't need to explain a diagnosis to the CMs. You only need to advocate for the need as it relates to the visit. There is more information about the GAC, and tips for helping sensitive kids cope, on the
Disabilities FAQ .
Sometimes we (parents of kids with special needs) get wrapped up in defining our child's needs and advocating for them so much that it becomes a habit, and we tend to have that "fight" on top when we are on vacation, too. This is not an IEP, or an assessment. It is a vacation! Relax, enjoy.
You do need to be aware that the GAC is not a Front-of-line pass, and that you will be required to wait, especially at busy attractions. Sometimes your wait in an alternate waiting area may be longer than it would be if you were going through the regular queue, and other times it will be shorter. And from long experience, I can tell you, getting the GAC and using it will not protect your child from stares or meltdowns. Many attractions do not have an alternate entrance -- you need to ask for assistance and direction from the CM at the entrance.
Another caveat... you need to be very nice to the CMs and follow directions. Sorry to say, there are people who do abuse the system, and it causes terrible resentment when it appears that people are trying to get special treatment, with or without a legitimate reason. Many CMs are on the defensive. You need to be prepared to wait, to distract your child while waiting, to pace yourselves. Never try to ride a busy ride twice in a row using the GAC, or even twice in a short period of time. And if the standby lines are manageable, please use them.
We worked very hard to get our son to be able to tolerate waiting in the queues -- it is a life skill. But we have no guilt about using the GAC when necessary, to prevent the kinds of meltdowns that can ruin your day.