Catalyzes the condensation of isopentenyl pyrophosphate with the allylic pyrophosphates to yield geranylgeranyl diphosphate (GGPP) which is a precursor of the ether-linked lipids. It is able to use dimethylallyl diphosphate (DMAPP), geranyl diphosphate (GPP), and (all-E)-geranyl diphosphate (E-FPP) as an allylic substrate. (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. Optimum pH is 5.8. Thermostable. Most of the activity for GGPP synthase remains even after heating at 60 to 70 degrees Celsius for 100 minutes, although 90% of the enzyme activity is lost on incubation at 90 degrees Celsius for 10 minutes. Isoprenoid biosynthesis; geranylgeranyl diphosphate biosynthesis; geranylgeranyl diphosphate from farnesyl diphosphate and isopentenyl diphosphate: step 1/1. Belongs to the FPP/GGPP synthase family.