By far the best gluten-free hamburger is at the Friends & Brgrs chain. The bun is completely incomparable, tasty and tender enough so that the fillings don't bubble out when you bite into it (as happens with many gf buns). The combination of sauces, steak and gf bun is so successful that it is difficult to find a competitor. It's rare to get such an excellent package when you have an eating disorder. A plus is that you can order from the vending machine in the chain's restaurants and the menu is also very clear.