Catalyzes the condensation of farnesyl diphosphate (FPP) and isopentenyl diphosphate (IPP) to yield geranylgeranyl diphosphate (GGPP) needed for biosynthesis of carotenoids and diterpenes. (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.