Step 1: Classify each option by tissue of origin to find the true epithelial neoplasm. Step 2: Carcinoid is neuroendocrine, gastric lymphoma is lymphoid (MALT), and GIST is mesenchymal from the interstitial cells of Cajal - none come from surface or glandular epithelium. Step 3: Gastric adenocarcinoma, by contrast, springs directly from the glandular epithelium of the gastric mucosa, making it the epithelial malignancy. Step 4: Its $Lauren$ subtypes - intestinal (gland-forming) and diffuse (signet-ring, leather-bottle stomach) - confirm its epithelial glandular nature. So the epithelial tumour of the stomach is gastric adenocarcinoma.\[\boxed{\text{Gastric adenocarcinoma}}\]