The two most common tumours affecting the stomach and intestines in cats are lymphoma and adenocarcinoma.
Adenocarcinomas can affect either the small intestine or the large intestine (rarely the stomach). They are usually quite rapidly growing tumours that often cause disease due to partial blockage of the intestine (loss of appetite, weight loss, vomiting and diarrhoea are the most common signs). Metastasis to local lymph nodes is common, and the tumours often invade the intestine quite extensively.
Surgical removal is the treatment of choice. Additional chemotherapy is used in some cases. Long survival times can be achieved in some cases (with surgery alone) even where the tumour has already spread to local lymph nodes so this can be worth considering, although not all cases will be suitable for surgery.