Please note that the GACs (Guest Assistance Cards) are NOT automatic fastpasses in any way, shape, or form. Once upon a time at
Disneyland in CA, these cards did work like "go to the head of the line cards." However, due to much abuse, Disney is much more careful now.
If you have a special need, you need to go to Guest Relations, explain exactly what the situation is and what type of accommodation the person in your party needs. Most CMs at most rides/attractions will bend over backwards to make you comfortable while you wait your turn like everyone else. They will provide a private waiting area, a shaded or cool waiting area, etc., but will not allow you to "fastpass" the attraction.
Sorry, don't mean to sound like I am being rude or anything here...this is just sort of a hot button topic for me. It seems that every time I visit WDW, I notice someone trying to abuse the system just to avoid waiting in a line. Relax and enjoy the magical view...lol...
For more information, you can visit the disAbilities board here on the DIS.