HI Well come to HCM I really appreciate your concern, the underlying cause of the gastritis is matter of concern, history of fresh blood during defecation is not due to gastritis or the diet taken (Garlic ) but it seems to be due to rectal lesion and it could be fissures or hemorrhoid these both need to be differentiated, both the condition may needs surgery, if the bleeding per rectum is matter of concern then the underlying cause of this is great matter of concern, and for that you will have to see the surgeon. Constipation may be the cause of this and if this is so then it is nothing to worry but try to relieve the constipation with drinking more water, fiber containing diet, you can even try stool softener like Isabgol powder. hope this information helps.