There's a great blog on how to make Japanese style hot/cold lunch boxes (bento boxes) with photos and recipes called Just Bento. Look for the recipe archive for ideas. Many of them are naturally gluten free, although, as with all asian recipes, you'll need to use GF soy and avoid using certain sauces (oyster, hoysin, blackbean, and most miso).
Also, there's a book coming out soon on Amazon from this blogger, so look out for that.
The spicy lentil snacks and potato dumpling things look particularly good: