We gave been going to B. Good for some time now -- they are a "healthy" burger restaurant. Although they don't have a specific gluten free menu, they have an allergen book with information about allergens, including gluten, and their burgers and fries do not contain gluten. Now, they are apparently stocking gluten free buns, so you can get a burger on a bun (instead of a lettuce wrap). They definitely have the buns at Legacy Place in Dedham, and in Coolidge Corner in Brookline. I asked the server in Brookline whether this was chain wide -- he thought it was, but you might want to call first if the bun is key! ) (Other locations include Harvard Square in Cambridge, Back Bay Newbury Street). Per the server, the bun is from Gillians bakery.
Editors note: The location in Brookline has closed. :(