Enables polypeptide N-acetylgalactosaminyltransferase activity. Involved in oligosaccharide biosynthetic process. Predicted to be active in Golgi apparatus. Is expressed in several structures, including adult head; embryonic/larval hindgut; embryonic/larval midgut primordium; foregut primordium; and hindgut proper primordium. Orthologous to human GALNTL6 (polypeptide N-acetylgalactosaminyltransferase like 6).