If you have needs that are not met by just having the wheelchair in line, then you would need a GAC.
But, if your needs are met by having the mobility device in line, then you don't need a GAC.
The designated 'mobility entrance' listed on the Guidemap for Guests with Disbilities is the entrance that can be used by someone with a visible need (walker, cane, wheelchair,
ECV, service dog, etc) without a GAC.
You can get these maps at Guest Services, at the park entry map kiosk and sometimes at your WDW resort ( you often have to ask because thy may be behind the desk, not with the other maps at the resort).
If a guest is trying to enter in another place, they will be either told they need a GAC or that is not an entrance. I've seen people try to come into the exit because they think that is the 'handicapped entrance' (I know some had been to other parks where that was the case and they just assumed Disney was the same.)
If you are told you need a GAC to use the entrance with a wheelchairs, my first step would be to check the Guidemap for Guests with Disabilities. If the entrance I was asking to use is the listed 'Mobility Access', I would show that to the CM. If they still say no, I would ask for their manager/supervisor. Doing those things usually clears things up.
We have been asked for a GAC a handful of times at WDW:
Many Adventures of Winnie the Pooh - we had Fastpasses for our whole group and the CM at the front of the line said she could not let us in with DD's wheelchair unless we had a GAC. I showed her our Fastpasses and was getting out the map which said "Enter through the Standard Queue or Obtain Disney's Fastpasses." As I was getting the map out, another CM overheard and told the first one that guests with Fastpasses need to be allowed into the Fastpass line with a wheelchair, GAC or not.
Splash Mountain - we waited in the regular line until the line reached a set of stairs. At that point, there is a gate with a wheelchair symbol that allows guests to bypass the stairs. We waited there for a while, then a CM came along and said we needed to continue in the regular line. I pointed out that we couldn't go ny farther with DD's wheelchair because of the stairs and that DD was in a wheelchair because she can't walk.
The CM said it didn't matter; she could not let us use the wheelchair gate because "that would be cutting". I asked her how she expected us to continue in the regular line with a wheelchair and stairs and she repeated the same thing.
At that point, I asked for her manager and the CM repeated the same thing to the manager, who basically said "What?! They can't go any farther! You need to open the gate for them."
The CM actually argued with the manager for a few minutes. The manager apologized to us and let us know the CM would be re-educated. The CM was still arguing as we passed thru the gate to the accessible waiting area ( where we actually did wait as long as it would have taken us to get on using the entrance with stairs if DD could walk).