Only time for a short reply, but most attraction queues, especially at AK and the Studio are wheelchair and
ECV accessible. You don't need a GAC to use the accessible line in those cases because it is the same line as everyone else is using.
Epcot and MK are older parks and were not built with accessibility in mind. They have some attractions where the main line is not accessible and you do need to use another access point. If that is the only point that is accessible, you don't need a GAC to use that access point. That doesn't mean the different access point will be faster. Just that it will be accessible.
WDW has been slowly adding accessibility to the main lines where they can. This is not to curb 'abuse' by people with wheelchairs/
ECVs, but is because the ADA requires 'Mainstream Access', rather than 'special treatment' for people with disabilities as much as possible.
Because of these changes in ride access, what was the only 'accessible entrance' on one trip for people with mobility needs may not be the only way in on the next trip. This can be confusing to people who have always used a particular way to get into a particular attraction, so check the Guidebook/Guidemap for Guests with Disabilities that you can pick up at Guest Relations for the correct entry point to use.
This is what is written in post #6 of the disABILITIES FAQs thread about GACs: (You can get to that thread with the link in my signature or you can find the thread near the top of this board.
Do I need a letter from the doctor?
No.
You don't need a doctor's letter and the CM is likely to not want to look at it because the letters are often not very helpful to the CM. Some people DO feel more confident asking for a GAC if they have a letter, but a letter is not required. According to the ADA (Americans with Disabilities Act) you can not be required to provide proof of a disability.
You can choose to show proof, but can't be required. If you do have a letter, it should reflect your needs, not your diagnosis.
(For example, a letter that says "My patient has xxxxxx and can't wait in lines. Please extend every possible consideration." is not helpful.)
There is a lot more in that post about GACs, including ideas on how to decide if you need one. I would encourage anyone who is using a wheelchair/ECV or thinks they might need a GAC to read it.
I wrote the information from our experiences spending about 20 days a year at different times at WDW. I had a lot of input, suggestions and information from CMs who work in Guest Relations at the parks in order to make sure the information is accurate. I recently had a CM review that post to make sure nothing had changed and she said it was still the current information/process.