Both my mother and brother have Celiacs. I recently brewed a G-free beer for them. It was a wit-bier of sorts, with coriander and orange peel, etc.
My homebrew store carries American Classic Sorghum Extract. You can get it here:
http://www.americanbrewmaster.com/catalog/product_info.php?products_id=1487
It ferments drier than what I'm used to, but the resulting beer still tasted ok. It definitely tastes different, but my mom and brother absolutely loved it (consider that they havent been able to have beer in ages).
A couple things to note:
1. You MUST use a
dry yeast. Dry yeasts are cultured on molasses and cane sugar, which is ok. Liquid yeasts are cultured in barley, so they are no good. Danstar assures me that all their dry yeasts are 100% gluten free (and kosher).
2. Cross contamination must be avoided. Replace all the soft plastics in your gear: hoses, stoppers, etc. They can absorb gluten from previous batches. Glass and metal and your buckets should be ok given you clean the bejesus out of them before you use them.
3. You must carb with sugar, NOT DME. Obviously, DME has barley, which in turn has gluten, so it's out.
There are other options for things to use as well. For the next batch I do, I think I'll steep some toasted buckwheat grains (also gluten free) to get a darker color and nuttier flavor. Buckwheat can be found at many nice grocery stores (Whole Food here has it). Rice is also ok to use. Fruit beers would also be mighty tastey. If you use something other than real fruit (extract or whatever), just be sure there aren't any gluten additives added (for thickening or whatnot. Not likely, but check to be sure).
PM me if you have any questions. I researched this extensively before I did my first batch.