I am not sure you would need a GAC for that, especially if your knee brace is visible (a visual cue that you do have a medical reason for needing the walkway slowed or stopped).
I am not sure how the GAC will be changing in WDW in the coming months, but you can always stop by Guest Relations and ask what you should do. Just tell them due to a knee injury, you wear a brace that requires you to have more boarding time, and have moving walkways stopped or slowed. If there is something they can do for you, they will.
When you go to enter a queue, ask the CM there what needs to happen for either the moving walkway to be slowed/stopped or for you to have extra boarding time. Occasionally here in DL (CA) the only place to slow a walkway is at the exit of a ride (which generally means going through the regular queue and then just going through a door to the exit).
For things like Rock'n'Rollercoaster, they will likely have you go through the regular line, through to the exit (literally through a door in the loading area) where a "limo" will pull up to give you more time. There are several ways in which they can assist you.
Again, I am not sure of their prevalence in WDW, but in DL they have been adding transfer cars, where the side of the ride vehicle either is cut out (like Haunted Mansion) or opens up. This was designed primarily to facilitate transfers for people in wheelchairs, but it sounds like this may help you as well.
This thread shows how some vehicles have been altered to help with transfers:
http://www.disboards.com/showthread.php?t=3093930
Also, please consider the amount of walking that is required for WDW - while your knee's range of motion is restricted, it will throw off your whole gait - this means everything from your little toe to your neck moves differently with every step. Walking could be more difficult than you have planned for. The average person walks 8-12 miles a day in the park - and they do it for several days in a row. You may want to consider if a mobility device would be beneficial - even a knee
scooter or a rollator, to give you a place to sit sometimes...