Golgi-resident glycosylphosphatidylinositol (GPI)-N-acetylgalactosamine transferase involved in the lipid remodeling steps of GPI-anchor maturation. Lipid remodeling steps consist in the generation of 2 saturated fatty chains at the sn-2 position of GPI-anchors proteins (PubMed:29374258). Required for the initial step of GPI-GalNAc biosynthesis, transfers GalNAc to GPI in the Golgi after fatty acid remodeling by PGAP2 (PubMed:29374258). Contains three transmembrane domains, including a tandem transmembrane domain insertion into its glycosyltransferase-A fold (PubMed:29374258). Transmembrane domain 1 functions as a signal for Golgi targeting (PubMed:29374258). The conserved DXD motif is involved in enzyme activity. Glycosylated. Belongs to the PGAP4 family.