Just from personal research, gastritis has many causes aside from bacteria.
Alcohol
Medicine (Tylenol, pain killers, antacids)
Processed foods (chemicals in food, pesticides, glyphosate)
Wheat (because of pesticide use and GMO)
Parasites
I would recommend building up your stomach lining through diet and/or supplements before adding MMS1. L-glutamine is sometimes used. I personally eat carnivore diet so lots of eggs, organic beef, and grassfed butter. I can definitely say this has made my skin thicker (I use B12 injections so I know my skin is much thicker). And of course, meat will help repair damage to your stomach lining. Meat contains amino acids including l-glutamine.
I would also see if you can do a food sensitivity test or a hair strand test. It might reveal a food sensitivity that could irritate your stomach lining.
I would try to eliminate possible irritants to your gut and try to build up your stomach lining prior to using MMS1.
MMS1 contains acid and supplying more acid to your stomach could be painful.
Another thing you can do is use a rife machine (also called a frequency generator). The cost is about $25. You would just place the electrodes on your stomach. It is capable of killing off various microbes and parasites.
I purchased a frequency generator from ali express (little white one--function generator DDS 100) that was $13.
You'll need:
--BNC cable with alligator clips
--electrode/ tens pads
(I purchased those off eBay.)
I used directions from Hulda Clark in her book The Cure for all Disease
www.scribd.com/document/291985758/Dr-Hulda-Clark-The-Cure-for-All-Diseases
--generator set to square waveform
--turn up amplifier knob
--set positive offset knob 1/4 of the way
--7 minutes "zapping", 20 minutes rest, repeat 2 more times
-- do daily sessions for 2 weeks or more, until no further improvement
She used 1500 or 25000 frequency. Her zapper on her website used 32000 alternating with 33000.
I personally used frequency 500000. (Be aware some electrodes can give small burns using this high of frequency.)
Hulda Clark's method using the "positive offset" allows you to broadly zap and kill viruses, parasites, microbes without needing the exact frequency for each individual bug. (You can place the electrodes over various body areas. Some areas may be too deep for it to fully penetrate. Hulda Clark does recommend using something internally as well as externally--ex. parasite tincture and zap the parasites with the frequency generator.)