1. GFO = a phosphate removing filter material, basically rust:
http://reefkeeping.com/issues/2004-11/rhf/index.htm
GAC is granular activated carbon. Lots of people run GAC, and I think it's a good idea.
2. Not as far as I know.
3. GFOs can cause the precipitation of calcium carbonate, which would lead to increased calcium and alkalinity dosing.