Cell wall formation. Catalyzes the transfer of a GlcNAc subunit on undecaprenyl-pyrophosphoryl-MurNAc-pentapeptide (lipid intermediate I) to form undecaprenyl-pyrophosphoryl-MurNAc-(pentapeptide)GlcNAc (lipid intermediate II). di-trans-octa-cis-undecaprenyl diphospho-N-acetyl-alpha-D-muramoyl-L-alanyl-D-glutamyl-meso-2,6-diaminopimeloyl-D-alanyl-D-alanine + UDP-N-acetyl-alpha-D-glucosamine = di-trans-octa-cis-undecaprenyl diphospho-[N-acetyl-alpha-D-glucosaminyl-(1->4)]-N-acetyl-alpha-D-muramoyl-L-alanyl-D-glutamyl-meso-2,6-diaminopimeloyl-D-alanyl-D-alanine + H(+) + UDP Cell wall biogenesis; peptidoglycan biosynthesis. Belongs to the glycosyltransferase 28 family. MurG subfamily.