just got back... experience with GAC

[SueM in MN]

I am thinking of renting a wheelchair, but for the times I can walk, do you think I would be able to get a GAC so that I would not have to stand during long lines? Or should I just sit in the wheel chair during the line wait?

Most of the lines don't have a place to sit.
For most of the attractions that load a "batch" of people at once, the line is still for a period of time - usually the length of the preshow - then it moves quickly for a few minutes while the next batch of people is loaded, then stops again for a while (Some examples are Haunted Mansion and Test Track).
For the attractions that load fairly continually, you are walking slowly during the whole wait - although some where the ride car stops for loading will have frequent short periods where you stop moving for 30 seconds to a minute at a time. (An example would be Pirates).
For the theater type shows, some have a few benches, but they are first come, first serve, so there is no guarantee that you will get a seat (unless you bring your own).

If I did get a GAC, do I need a letter from my doctor, (or maybe I can just show the CM my foot long scar on my leg).

No, you don't need a letter. just to be able to explain to the CMs in Guest Services in the parks what your needs are. But, your needs are basically endurance.