Steak 'n Shake is a bit of a challenge, but depending upon your sensitivity, here are a few suggestions: cottage cheese, fries, cheese fries, burgers without the bun, shakes, salads, and sodas. Their breakfast menu is also good if you stick to eggs, potatoes, bacon, sausage, etc.
I LOVE this restaurant!!! Why? Cuz *unlike* McDonald's, the fries at Steak N Shake *are* gluten-free!!!!!!!!!!! I take off the bun of their steakburgers, and eat them too; NO fillers in the hamburger patties. Also, because I eat low-FODMAP, knowing there are no onions, garlic, or other spices added to the meat that would cause problems...is huge!! They don't have fillers and junk used in their meat; a rarity in chain restaurants nowadays. I'm seeing no problems from eating their burgers without a bun....so I eat their fries with their burgers (bunless).....and it's awesome food!!! Highly recommended.