Prenyltransferase that catalyzes in vivo the transfer of the heptaprenyl moiety of heptaprenyl pyrophosphate (HepPP; 35 carbon atoms) to the C3 hydroxyl of sn-glycerol-1-phosphate (G1P), producing heptaprenylglyceryl phosphate (HepGP). This reaction is an ether-bond-formation step in the biosynthesis of archaea-type G1P-based membrane lipids found in Bacillales. all-trans-heptaprenyl diphosphate + sn-glycerol 1-phosphate = 3-heptaprenyl-sn-glycero-1-phosphate + diphosphate Membrane lipid metabolism; glycerophospholipid metabolism. Homodimer. Belongs to the GGGP/HepGP synthase family. Group I subfamily.