The Free From burger buns from Tesco are pretty okay. So are the GF rolls from M&S. If you very lightly toast the bread, that apparently makes the bread less minging for sandwiches. A shedload of mayo also helps, if you're doing something like tuna, lettuce and mayo.
Sushi and rice balls are a good one.
Chicken salad, potato salad, pasta or rice salad can be taken in small pottles.
Also, doing something like a selection of cheese, GF crackers, ham, salami, cold chicken, grapes, tomato, cucumber, sweet peppers, cold boiled eggs., yoghurt.
There's always fritata, if your DC will eat that, or making a crustless quiche.
GF
Cold GFpizza.
GF savoury muffins.
If you're baking, heartily recommend the GF flour from M&S, just exchanging it in recipes 1c for 1c of standard. It's fantastic.