Can a long does of amoxicillin cause gastritis and how long can it last after a two weeks of taking the antibiotic?

Yes: all antibiotics can cause various forms of GI upset. They also change the balance of bacteria/micro organisms in the GI tract which can cause symptoms. Often eating yogurt daily with live cultures or probiotics can be helpful in alleviating the symptoms. if this does not help after a week i would recommend talking to your primary care doctor. I would also avoid alcohol during this time.