Catalyzes the condensation of isopentenyl pyrophosphate with the allylic pyrophosphates to yield geranylgeranyl diphosphate (GGPP) which is a precursor of the ether-linked lipids. (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. Isoprenoid biosynthesis; geranylgeranyl diphosphate biosynthesis; geranylgeranyl diphosphate from farnesyl diphosphate and isopentenyl diphosphate: step 1/1. Belongs to the FPP/GGPP synthase family.