My husband tore his knee while we were on vacation in
Disneyland. (Not at the Parks!) We called and had a wheelchair delivered to our hotel for about $59 plus tax for 5 days. It was not expensive, and honestly it was a saving grace: it made our travel to and from the park MUCH easier. (And we were staying ffairly close to DLR at the HOJO.)
We did not ask for a GAC at either Park- in DL the castmembers could see what accommodations he needed, and he did not need any special assistance of any kind. (The castmember usually asked if he could get out of the chair and get into a ride.) There were several rides in DL that you will enter through the exit and be put on the ride from the exit area. It was a little awkward and backwards at first! (It was also a little embarrassing to bypass a waiting line and be able to ride in a few minutes. I felt uncomfortable over it a few times, at Nemo especially.) In DCA, lines are set up differently, so you may find you would like to use a GAC over there. We just used Fastpass and adjusted our expectations.
Also, if you decide to rent a wheelchair when you get to the Parks, know that you cannot take those wheelchairs out of the Parks and into Downtown Disney. That's another good reason to rent a wheelchair offsite! Plus, during busy times, they can run out of available wheelchairs.
You could rent a offsite manual wheelchair, go to the Parks and rent an electric wheelchair from Disney and the castmembers will be able to store the manual chair for pick up, so you can collect it and go to Downtown Disney for dinner or a movie.