Gluten free beer must, by law, contains 20 ppm or less of gluten. The newest, and best process to keep the natural taste of your beer is to use normal ingredients and then introduce a process to reduce the gluten content typically using an enzyme at the start of the fermentation process to break down the gluten protein.
The original method was to use a malt from naturally gluten free cereals or pseudocereals such as sorghum, millet, buckwheat, rice, quinoa or maize. These beers often have slightly different aromas and flavours to regular beer.