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. To a much lesser extent, is also able to use geranyl diphosphate (GPP; C10) and geranylgeranyl diphosphate (GGPP; C20) as the prenyl donors, but not farnesyl pyrophosphate (FPP; C15). Cannot use glycerol-3-phosphate (G3P) or 3-phosphoglycerate (3PG) as an acceptor. all-trans-heptaprenyl diphosphate + sn-glycerol 1-phosphate = 3-heptaprenyl-sn-glycero-1-phosphate + diphosphate Membrane lipid metabolism; glycerophospholipid metabolism. Homodimer. Cells lacking this gene show a cloggy growth and do not produce the dephosphorylated and acetylated derivatives of HepGP. The HepGP product, which is the first archaea-type G1P-based ether lipid being identified within the phylogenetic domain of the bacteria, was found to be subsequently dephosphorylated and acetylated in vivo. However, HepG and its acetylated derivatives represent only a minor fraction of the total lipid in B.subtilis. Belongs to the GGGP/HepGP synthase family. Group I subfamily.