Limited options beyond snacks, but if you are pressed for time (less than an hour before your flight), this is likely your only option. One of two places in the airport that can accommodate gluten free according to the information desk. The GF sandwiches are made in a GF facility.
The sandwiches are on rice bread that is extremely dry and disintegrates upon opening the packaging. You will not be able to choke it down without condiments or swigs of water with every bite, it is that dry.
I didn’t get sick from the food (a couple of side dishes the first time. Thankfully I missed the sandwiches the first time through), so there is that. A sandwich and a drink will be equivalent to the cost of a lunch at a normal sit-down restaurant. At least this has some options. With the huge number of restaurants at O’Hare, you would think there would be more options, but it seems like the majority have a vendetta against GF.