That doctor who told you gastritis has nothing to do with food allergies is full of hogwash.
Food allergies can cause gastritis, and gastritis can make food allergies worse or even lead to more food allergies (because food can leak through the inflammation into the bloodstream, increasing the chances of developing antibodies to the food).
Are you taking a high quality probiotic? If not, it would be a good idea because one side-effect of inflammation and reduced stomach acid (Zantac reduces stomach acid production) is the increased chance for developing yeast overgrowth.
Yeast overgrowth can cause the sensation of fullness you describe.
Lyme/coinfections can also cause the fullness sensation you describe.
A better treatment is a combination of Vitamin B12 (get methylcobolamin), DGL (take 20-30 min. prior to meals), marshmallow root, and probiotics.
Ginger tea is also helpful as it can reduce inflammation and also soothe nausea and indigestion (if no gallbladder problems). Peppermint tea may also help. It is best to make your own tea from the bulk herb (ginger root or peppermint leaf), as the pre-made teas often have other ingredients that may not be helpful.
For Ginger tea, I put about
an inch of fresh ginger root (peeled) in 2 cups water, bring to a boil, then let it sit with the lid on but the stove turned off for about
20 minutes.
For Peppermint tea, I put a tablespoon of the dried peppermint leaf in a tea ball. Bring 2 cups of water to a boil, then toss in the tea ball and turn off the stove. Let it sit for 5-10 minutes with the lid on, then pull out the tea ball.
The longer you let the herb stay in the water, the stronger the tea will become. If the tea is too strong, you can add plain water to dilute it after making the tea.
Might also be good to avoid spicy foods, tomatoes, citrus, vinegar, and any other foods you know cause problems or to which you are allergic.
Turmeric is another good anti-inflammatory herb that many find helpful for all sorts of Lyme-related issues. Lyme is good at causing lots of inflammation in the body.
Oh, and have you been tested for H. Pylori? That bacteria is also known to cause a lot of issues with the stomach, including inflammation or even ulcers. Blood tests are not accurate - a breath test, stomach biopsy, or aspiration of fluid from the stomach are the most accurate ways to be tested.
I hope this helps...take care,