There are two types of charcoal that you may come across as a coeliac or gluten intolerant person. One is the type used on some barbeques (BBQ's) the other is a stomach medicine 'activated charcoal' otherwise known as activated carbon or activated coal. Both versions may pose risks.
The barbeque version is often made out of starch and because it is inexpensive the starch can be wheat starch which then permeate food cooked above it or on occasion may get brushed with minute amounts of it. The other which is activated charcoal, is made from a variety of sources so if you are a keen user then it is wise to check to see whether wheat starch is one of the sources that is used on the brand that you are choosing.
Here are a few of the links that question using charcoal: