You won't find a super extensive range anywhere, but MegaMart in the Centre near Ramada Junction has by far the best range I've seen in Doha. They have pastas, gluten free flours, oatcakes and, in the chiller cabinet- a pretty good selection of baked goods- bread products and cake-y things. Bateel Bakery in West Bay (in the West Bay Petrol Station Plaza) has a pretty palatable gluten free bread.