Is it possible that the left supraclavicular lymph nodes are swollen if you suffer from gastritis? (hard and painless). the only thing you read is that the left supr lymph nodes swell with malignancies?

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. Gastritis would not cause enlargement of the nodes in the neck. You may feel the nodes every couple of weeks, if any of the lymph glands doubles in size, becomes painful, ulcerates, or other nodes become enlarged, or you develop fever, or lose weight, you should see a doctor.

Gastritis is not commonly causing swollen lymph nodes, they are commonly reactive to inflammation in the vicinity, best to have an exam with your doc for peace of mind.