don't think there is a specific store in Doha that has this type of food.  You will find bits and pieces in various stores around town.  MIght want to try MegaMart in The Mall (behind the Ramada).  The Lulu across D-Ring from the Landmark does have a small fresh organic fruit / veggie section and a small health food section that does have gluten free rice flour.  Expect to pay lots of money though.