I had a wonderful experience on the ship eating GF. The only time I wasn't totally pleased was at Remy, they were less accomidating than the MDRs. In the MDRs, they made anything I wanted gf, with the exception of some desserts (this was the only spot that was not great, but I was ok with what was offered).
It was much easier to go to the sit down breakfasts/lunches than it was the buffet. Just easier to order and let them deal with it rather than having the chef at Cabanas tell me what I could/couldn't eat, etc.
As for excursions, those are run by outside companies and I found that they were not very accommodating (a lot of people have no idea what gluten is). I'm not super sensitive, so I just ate what looked ok and was careful. But if you're Celiac or sensitive you'll want to bring some pre-packaged safe snack foods to tide you over or just book shorter excursions.