Catalyzes the second step of glycosylphosphatidylinositol (GPI) biosynthesis, which is the de-N-acetylation of N- acetylglucosaminyl-phosphatidylinositol. {ECO:0000269|PubMed:14742432, ECO:0000305|PubMed:10085243}. This is the function of ENSG00000108474 (PIGL, phosphatidylinositol glycan anchor biosynthesis class L).