Enables beta-1,3-galactosyl-O-glycosyl-glycoprotein beta-1,6-N-acetylglucosaminyltransferase activity. Involved in response to insulin. Located in extracellular space. Orthologous to human GCNT1 (glucosaminyl (N-acetyl) transferase 1). [provided by Alliance of Genome Resources, Apr 2022]