Raye's Mustard is a small mustard mill in Maine. I visited it once when my wife and I were visiting Maine and we picked up some mustard. They have all types and they are all delicious. They make great gifts, too.
I had not had some in a while and wondered whether or not it was safe to eat - so I contacted them. They let me know that almost all of the mustards are gluten free. They are kosher certified and the non-gluten free ones are produced on a separate line.
The non-gluten free ones are the beer mustards (and these are not kosher certified and therefore produced on a separate line from the other mustards). She even checked with the company that supplies their wine - and the wine is gluten free - but it is not kosher certified and therefore made on the same line as the beer mustards, so there is a risk of cross contamination (I would avoid it).
So if you stay away from the beer and wine mustards you should be fine. I have had no problems. The Down East Schooner is one of my favorites. This is how mustard is supposed to taste. Old World Gourmet is good too.
They also have 4 packs and 6 packs which make great gifts.
Cook up a hot dog on a GF bun and spread on the mustard,
The Un-Gluten Guy