When I went to the Disney Studios earlier this month to get a GAC, I did have my GAC from our previous trip. The CM took it backstage & brought me out a new one. I asked about the old one, & she said they keep it. For my personal security, I don't like that, but I wasn't thinking fast enough at that time. Next time I will not necessarily turn over an old (dated) GAC. I prefer to destroy it myself.
In all the times I have gotten GACs over the last year & a half, no one has ever mentioned anything about proof of any type. Interesting though because I was talking to a coworker just last week about his getting a GAC on his family's upcoming trip. Someone mentioned he should get a note from the doctor & I said it wasn't necessary. He can always show his incision - he is a heart transplant recipient.