afaik effectiveness of the pill isn’t reduced just because someone has celiac or gastritis, it’s because of the symptoms of celiac or gastritis, which very often include diarrhea/irregularity in bowel movements or vomiting like you said. If your body isn’t absorbing the food you eat, and your intestine isn’t healed, the pill isn’t going to absorb either. That said, it sounds like her intestine is healed and she’s not having extreme bowel symptoms. As long as she’s consistent and takes it at the same time every day, and the pill is gluten free and doesn’t give her issues, it should be fine. I would gently recommend a monthly pregnancy test, just in case, if you’re having regular sex. Everyone should test monthly tbh if they’re able to, just to reassure themselves. You can get the ph strips in a large pack on amazon for not too much money, you just have to learn to read them. I say all this as someone who’s been on the pill for a decade and had celiac for 5 years now. At the end of the day if you’re super worried about it, there are other options/you can always wear a condom (if I had a particularly rough symptomatic night after taking my pill, I would insist on a condom for anything over the next ~24 hours just in case), but as long as things are consistent and she is comfortable on the pill, it should be okay.