The United Center website clearly tells you which concessions have gluten free. Seems like it changes so I'd check the website.
Concession I went to had only two items, both listed on website as GF. When I asked if this was the gluten free options I was told that they only had the 2 items at that counter.
— Somewhat
— Gluten-free items are marked on the main menu
— Not Much
— Average