Hello,, it's pretty common in people feeling sleepy after having food or full stomach, but if you practice sleeping after food it will become habit and you can't resist that. Don't worry it is not a disease change your habit,, always eat little less than full stomach,, take a 20min walk after each meal instead of sleeping follow these practice, it will become habit. And you won't get sleep after food and walking makes you more healthier. For gastritis avoid outside food or spicy food or junk food, eat timely drink more water.