Some of these lists of 100% gluten free restaurants contain information that is outdated, so keep that in mind before you plan a major trip based on this information!
Of course, if you do go to one of these restaurants, and it checks out, please share it so we can all find it more easily!
This is a current list with 39 restaurants in the US (14 states) and 2 in Canada. All but three are 100% gluten free, and the remaining restaurants (identified in purple) are close to being 100% gluten free. Almost all the restaurants have reviews (or video reviews), so you can decide for yourself. Check out GF Life 24/7 for more travel tips including gluten free in Disney!
This is a link to current lists for several areas in London and includes many 100% gluten free cafes, restaurants, and bakeries. (The 100% gluten free links are in the gray box.) The Coeliac Plate is a great resource for traveling in the UK. Check it out!
This 100% gluten free list is from 2014 and has about 100 restaurants and bakeries in 33 states, plus 11 restaurants in Canada, 2 in the UK, and 1 in Italy. (From a quick glance, it looks like a lot of bakeries.) Direct link to PDF.
This is a comprehensive list of 100% gluten free restaurants, with 59 restaurants in the United States, 33 locations of a single restaurant in Mexico (!), and one restaurant in New Zealand. Some of entries are from 2010 or 2011, so be sure to check out the restaurants before you go. (For example, I know Pica Pica Maize Kitchen in Napa is closed (but the San Francisco location is still open!)
(Scroll down until you start to see the listing by state.) This site is also somewhat dated, but has listings for 116 restaurants in 32 states. Be sure to scan the comments for a number of additional restaurants that have yet to be added.
Please let us know if you find any additional sources of 100% gluten free restaurants!