I went with a group of friends to Phuket last month (and we often travelled into Patong to spend the day there). I'm coelliac and never found any trouble there. I mainly just stuck to dishes with rice (i've nevered liked rice noodles, tried them over there and my verdict was still the same). None of us ever eat at the street stalls, i know people say most of them are fine but when we got there and saw the hygeine standards common sense kicked in and we just stuck to places that looked clean. The good thing is every place we went to had menus in english and gave you an idea of what the dish had in it (if you go to a place with lots of tourists chances would be it'd be in english)... Its suprising how much of it you can eat! I never worried about the gluten free cards, honestly, the english in most of the places outide of the resort was pretty basic so would just over complicate things.
A good place to eat in Patong was in the shopping mall called "cafe 101", there were plenty of dishes there that you could eat, was very clean and was super fast! They even said as we were leaving "ok, we see you tomorrow" (it was our favourite place and went there quite a bit!)
A tip for the plane ride over would be to chuck a snack in your bag once you get through security, the meals weren't too flash on our flight and on a 9 hour flight, you tend to get a bit hungry!
I wouldn't be concerned what so ever if i were you. On a lot of the day tours we went on they provided snacks and lunch. Everthing is so fresh over there and you can actually see what was in the food so can make a judgement on what you can and can't eat. They say that the local soy sauce is fine, when i went on the tours and the dishes ih soy sauce i didn't feel any effect. i didn't worry about bringing foods on those trips, i all else fails, they'll have fruit! Note though, i can tolerate a tiny bit of gluten so this could be why i didn't feel ill. I don't want to alarm you if you're a strict no gluten person but just still be aware of what your eating, ifyou think i might have gluten, stop eating eat (same goes for if you think that something could give you food poisoning, if it tastes funny chances are its probably no ment to taste like that - no one wants to get sick on their holiday!)
Well i hope this helps and you'll have a blast over there!!