Part of the complex catalyzing the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to phosphatidylinositol, the first step of GPI biosynthesis. a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + UDP-N-acetyl-alpha-D-glucosamine = a 6-(N-acetyl-alpha-D-glucosaminyl)-1-phosphatidyl-1D-myo-inositol + H(+) + UDP Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Belongs to the PIGP family.