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