Enables acetylgalactosaminyl-O-glycosyl-glycoprotein beta-1,6-N-acetylglucosaminyltransferase activity. Acts upstream of or within intestinal absorption; kidney morphogenesis; and tissue morphogenesis. Predicted to be located in Golgi apparatus and membrane. Predicted to be integral component of membrane. Orthologous to human GCNT3 (glucosaminyl (N-acetyl) transferase 3, mucin type). [provided by Alliance of Genome Resources, Apr 2022]