Involved in biosynthesis of the polyprenyl side-chain of methanophenazine, an electron carrier utilized for methanogenesis. Catalyzes the condensation of isopentenyl pyrophosphate with the allylic pyrophosphates to yield geranylfarnesyl diphosphate (GFPP). It prefers geranylgeranyl diphosphate (GGPP) and farnesyl diphosphate (FPP) as allylic substrate. (2E,6E,10E)-geranylgeranyl diphosphate + isopentenyl diphosphate = (2E,6E,10E,14E)-geranylfarnesyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. Optimum pH is 7. Optimum temperature is 40 degrees Celsius. Homodimer. Belongs to the FPP/GGPP synthase family.