Hmm...I just googled and it says to avoid wheat, rye, barley and oats.
That's pretty damn challenging in terms of a beer recipe, isn't it?
Have you checked out http://www.mrgoodbeer.com/gf/
In any event. I imagine it would mean something like the foillowing, assuming 'GFG' stands for some gluten-free 'grain' like buckwheat or millet:
1) Buy a lot of GFG.
2) Soak it in water.
3) Sprout it briefly.
4) Roast/dry sprouted GFG. (Now you have malted GFG! )
Now you do the normal steps of an all-grain brewing. Grind it. Mash it. Sparge it. Boil/hop it. Chill it. Ferment it.