Technically, you do not need a GAC if you use a wheelchair, but I have always found that a GAC makes things run smoother, if only because it helps the CMs know what type of accomodations you need. Really, few people are going to go to the hassle of getting a wheelchair just to utilize a different entrance, etc, but I know that many of us have seen groups of rowdy teens( I have 2 of my own in that age group, so I'm not picking on anyone
) racing through the parks pushing the chair and the next time you see them, one iof the other kids is sitting in it and the one that was riding is now running- I think that was a lot more common in the past than it is now since the introduction of GACs. Some people are mobile enough to be able to stand/walk onto a ride and there is a special place to park your wheelchair- so a GAC would help the CM know that you need to use the FastPass line or to have accomodations in the regular line- something they would not know if they didn't see you in your wheelchair.
I've done it both ways and wouldn't attempt to tour the parks without a GAC any more- your health and comfort come first and if getting a GAC can save you any aggravation then it's well worth the few minutes that it takes to stop at Guest Services and get one.
Enjoy your trip!!