What is the difference between gastric cancer and gastritis? ( like how can you tell if you have gastritis and not g.cancer. ) like a symptom or something to make the difference?

Gastric cancer is relatively rare and would be very unlikely, especially in a 25 year old. There would be no way to diagnose either of them definitively based on symptoms alone, and both of them require testing to establish the diagnosis. If you are having upper G.I. symptoms, do a short trial of antacids, and if you are no better, see your doctor or a gastroenterologist. Good luck.