I had IBS for many years, what helped was separating food items, like fruit in the morning only, never with other food.
Little to no grain.
Little to no milk, but butter and heavy cream is good, so are unpasteurized cheeze.
combination protein and veggies was tolerable, but not protein and starch.
yoghurt was a disaster, unless taken plain with no other food for a couple of hours.
Probiotics never helped me.
Iodine is normally not irritating to the stomach, but when having gastritis, almost everything is.
What helped me was enzymes, vit B and zink, sulfur (MSM)
borax and iodine. (borax I believe would be very soothing for your gastritis; it is to all kind of burning)
One way to take Iodine
without problems is to take it with vit c (natrium ascorbate in your case, or magnesium ascorbate)
If you have access to borax, I suggest to take your Iodine
in juice or water with a pinch of vit C, and a good pinch or two of borax. + magnesium citrate.
Capsule it if the taste is not to your liking.
These four work synergistically, and the most gentle way for my once ever so sensitive stomach.