Get whatever vegetables are in season. Try to keep them similar, such as hard root vegetables (carrots, parsnips, beets, sweet potatoes, onions), or nice tender spring vegetables (asparagus, sugar snap peas, yellow squash). Cut them into similar size pieces, toss in a little olive oil, unrefined salt and Balsamic vinegar. Roast at 400 degrees until they are caramelized and tender. Toss them with cooked, drained brown rice pasta, a little more olive oil, and grate real Parmagiano Reggiano cheese over it all.
Are eggs allowed? A spinach frittata would be nice.
How about a ratatouille without tomatoes?
A nice soup that would fit your criteria would be pasta e fagiole (pasta and beans) Here's a link: http://chowhound.chow.com/topics/661138. In the replies, about half way down is a post from a cook named Gio who posts an excellent tomato-free recipe. Just use gluten free pasta.
Make a kiwi, strawberry and spinach salad (just google that and you'll find lots of recipes). Add almonds or pecans, and mandarin orange segments.
Whole Foods makes a delicious salad that contains edamame, blueberries, cashews, chopped kale, and red onion with a pomegranate or acai juice and olive oil vinaigrette.
You know what I sometimes do for inspiration? Browse a market like Whole Foods, in their prepared food department. You will see acorn squashes and portobello mushrooms stuffed with amazing things, and interesting salads, and many gluten free or vegan options. I don't buy the prepared food, but I will buy the ingredients or a variation of them, and go home and make the food myself! You might find some great ideas for your gluten free vegetarian get-togethers.