Branching enzyme that converts linear into branched poly-N-acetyllactosaminoglycans. Introduces the blood group I antigen during embryonic development. It is closely associated with the development and maturation of erythroid cells. a beta-D-Gal-(1->4)-beta-D-GlcNAc-(1->3)-beta-D-Gal-(1->4)-beta-D-GlcNAc derivative + UDP-N-acetyl-alpha-D-glucosamine = a beta-D-Gal-(1->4)-beta-D-GlcNAc-(1->3)-[beta-D-GlcNAc-(1->6)]-beta-D-Gal-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + H(+) + UDP Protein modification; protein glycosylation. Belongs to the glycosyltransferase 14 family. Gcnt2 - I branching beta-6-GlcNAcT2, variant 3 (IGnT) Gcnt2 - I branching beta-6-GlcNAcT2, variant 1 (IGnT)