A GFI outlet should be on it's own circuit, at least that is the building code here anyway. Found out about it when I built and wired my garage. Inspector was a decent guy and passed it, but he told me that I had to put it on its own circuit.
That sounds weird to me, so you'd have to have a separate breaker and a separate run of wire for every outlet?
I believe that when I built my garage(25) or so years ago, in Colorado, I thought the deal was the first outlet in a string had to be gfi, and that would protect the rest of the outlets on that line. I could be remembering this all wrong, its been a while.
