I'm really not sure about the GAC question (DD uses a wheelchair, so we don't need/qualify for a GAC at any park). However, I did have two thoughts. First, I know the Express Pass can be very expensive (I think the cost varies depending on the time of year), but if you can swing it, they are VERY nice to have. Second, if your DS is 6, there might be a lot of rides that he won't be tall enough to go on. Universal seems to have many more rides tailored to older teens and adults than Disney does. Depending on the height of your kids, and if you're going to be splitting up for rides, you might be able to just get the Express Pass for your other child and one parent.
Sorry I don't have any info on the GAC, but I'm sure someone else here will know.
Have a nice trip!
Mary