Alpha-1,6-mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers the second mannose to the glycosylphosphatidylinositol during GPI precursor assembly (By similarity). Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Not N-glycosylated. Belongs to the PIGV family.