I would be interested to hear from you what causes your gastritis. As far as I know, stress can cause gastritis (like ulcers), if you 'eat up' your anger and frustration. Otherwise of course coffeine and irregular meals, etc, but you're probably avoiding that anyway because of IBS. Then of course medications on an empty stomach are a main ofender.
I think once you're on a stronger medication, you can basically eat 'normally', all the really bad things we're avoiding anyhow with Heather's diet apart, perhaps, from white meals, etc (altogether, if we weren't sufferering from IBS, a whole grain diet and raw veggies would probably be better for our stomachs)
Well, take care, sorry for you to have this, the symptoms are - almost - worse than IBS! But fortunately it's easy to cure!