The person who wrote about no GACs during the summer was misinformed.
If you haven't already checked it out, look at the disABILITIES FAQs thread which is located near the top of this board. There is one whole section of FAQs on GACs (Guest Assistance Cards).
We have been at WDW at various times, including Easter and the time between Christmas and New Years (some of the very busiest times of the year). We have been able to get GACs for DD on all those trips.
When you email WDW, they will not tell you about GACs because (as others mentioned), a GAC is not given based on the diagnosis, but based on needs. People with the same diagnosis can have very different needs - some people with children with autism have posted here that having a GAC was vital to meeting their needs. Other people whose children also have autism, have posted they did not need a GAC at all - so needs are variable.
Also, if you email WDW, they are not going to promise anything - you need to go to Guest Relations at the park and actually explain your child's needs (with the child there).
Another thing to think about is that even if you do have a GAC and it helps with lines, it will not help with things like generally how busy the park is, or lines in restrooms or restaurants. Even during the generally busiest times in the parks, there are particular days and times of day that are quieter for each park. Knowing some of that information and planning around it may be as helpful (or even more helpful) than having a GAC.