Prenyltransferase that catalyzes the transfer of the geranylgeranyl moiety of geranylgeranyl diphosphate (GGPP) to the C3 hydroxyl of sn-glycerol-1-phosphate (G1P). (2E,6E,10E)-geranylgeranyl diphosphate + sn-glycerol 1-phosphate = diphosphate + sn-3-O-(geranylgeranyl)glycerol 1-phosphate Homohexamer. Belongs to the GGGP/HepGP synthase family. Group II subfamily.