I do not have celiac disease, but my fiance does. He was diagnosed in Australia years ago. It presents a challenge, but there are actually a myriad of food sources out there.
Stores selling GF items include as mentioned Marks and Spencers, Little Giant and Health Gate. Also Taste (upscale Park n Shop, last time I looked it had GF pasta among other things), Wellcome (GF rice/buckwheat pasta in some (not labelled as such, but ingredients are rice or buckwheat and water, in an orange bag, shell and macaroni shapes), some stores also sell GF corn flakes)), Gourmet, City Super, Great, Three-Sixty, Oliver's, Market Place by Jason's, Jusco, Healthy Delight, Green Earth Society shop in Sai Kung, and others if you look closely (i.e. Wing-on has Blue Diamond Nut Thins, many organic stores will sell GF items, I saw Pamela's products in one in Mid-levels). Some of these places group items together, most do not. Some also have special tags on the shelves to indicate an item is GF.
Plus there are the normal Asian foods that are GF - including but certainly not limited to rice, rice noodles, rice crackers, mung bean noodles, sweet potato noodles, pure buckwheat soba noodles (be careful and read the ingredients because most also have wheat flour in them), papadum, rice paper rolls, and various flours (rice, sweet rice (aka glutinous rice, but no gluten), tapioca, chestnut, corn, garbanzo, etc for baking).
As for restaurants, it does depend. We haven't bothered inquiring about GF items in Chinese restaurants, we just order what we think will be GF (same for Thai, Vietnamese, Indian, Japanese, etc). I have a link to a Mandarin Chinese restaurant card, which looks like it has simplified characters but still might be understandable to local chefs. There's also a Cantonese card (PM me for link), which seems to have a much different format, not sure why. And there's a link to a handwritten card that seems to have worked for the person who posted it (PM me for link). My fiance isn't super strict, so he does eat soy sauce and isn't too careful about sauces in dishes. We're usually able to find something for him to eat at most any type of restaurant.
Taco Truck in Quarry Bay makes their own corn tortillas out of pure masa harina (no wheat flour). Munch in Knutsford plaza has buckwheat pizza, but we usually make our own at home (if your flat doesn't have an oven, you can get a microwave cum grill cum convection oven or a tabletop oven). Ali Oli in Sai Kung sells a GF flour mix and has GF items in its freezer section, but I find it cheaper/easier to use a breadmaker and/or our oven. Ruby Tuesday has GF items marked on their menu (so I've been told), as does Life Cafe, Deli Lamma (on Lamma Island) and the Royal HK Yacht Club restaurant. Apparently the owner and staff at The Flying Pan can tell you what items are GF, although it's not marked on their menu. There's a new place called Green Waffle in Central that says they will be serving GF waffles soon (possibly by this week). La Creperie in Wan Chai serves pure buckwheat gallettes and great cider from Brittany, though pricey. Mosburger has "rice burgers" but my fiance says they probably use soy sauce in the patties. And Toi Hokkaido Rice Pizza has a rice "crust" although it falls apart easily.
Unfortunately, I know of no local group dealing specifically with celiac disease. There is a local allergy group which covers food allergies (PM me for link), but that section of their site is only in Chinese. As for testing, a gastroenterologist should be able to do that for you after taking a thorough history and physical. Consulates often list local doctors on their site (the U.S. one lists two GI docs). They should order at least three tests: Total IgA; IgA antitissue transglutaminase (tTG); and IgA antiendomysial antibody immunofluorescence (EMA). If you're already on a GF diet, the results may not be conclusive. If the tests indicate celiac disease, they should do an intestinal biopsy to confirm the diagnosis. If you are diagnosed with celiac disease they may do other blood tests to look for various deficiencies.
Thanks for starting this thread Jason, I agree that a good repository for information is much needed. Great tip about the GF sauce in KC! And thanks to everyone for sharing what they know about eating GF in HK, we're always on the look out for new opportunities. Please feel free to PM me if you have any questions!
Aileen