I've personally found that finding non-perishable foods helps me immensely here - I don't know about gluten-free specifically, but cliff bars, trail mix, dried fruit, and nuts all work well for me. I like them because I can just toss them in to my bag and forget about them, or store them where I'll be if I have a locker/desk. As long as I remember to grab more when I run low, it means I'm never out of food, and my days go much smoother.