My son age 12 also has difficulties waiting in line and we have used the GAC in the past, but we also plan for a big break in the afternoon at our hotel within walking distance. With all the sensory stimulation, my son usually lasts about 3 to 4 hours in the park in the morning, we swim in the afternoon, and then go back to the park. We also really utilize strategies like getting there when the park opens and lines are at their shortest, and really understanding how to get the most of our fastpasses.
The GAC we have been given is an "alternate entry" one that on most rides with FP available, we are just directed to the fastpass line. On other rides, we are directed to the wheelchair line-sort of- sometimes we are asked if we can do stairs in some and shuttled into another line. Although it is not a Front of the the line pass, our experience has been that it has shortened lines for us, with few exceptions.
We have had very positive experiences with Disneyland and their willingness to accommodate for my sons needs. It has been a huge help! We don't use the GAC all the time, but when we need it, it makes our trip doable.
(now if I could only figure how to work WOC, Aladdin, and Fantasmic! without long waits! And without paying an arm and a leg-that is my current dilemma!)
You will find other info on the DISabilities forum on this site.