Geranylgeranyl pyrophosphate synthase; part of the gene cluster that mediates the biosynthesis of paxilline, a mycotoxin that acts as an inhibitor of mammalian maxi-K channels (PubMed:11169115, PubMed:23949005, PubMed:16494875, PubMed:19529962). PaxG, the geranylgeranyl diphosphate (GGPP) synthase is proposed to catalyze the first step in paxilline biosynthesis (PubMed:23949005, PubMed:16494875, PubMed:19529962). Condensation of indole-3-glycerol phosphate with GGPP by paxC then forms 3-geranylgeranylindole (3-GGI), followed by epoxidation and cyclization of this intermediate (by paxM and paxB) to form paspaline (PubMed:23949005, PubMed:16494875). Paspaline is subsequently converted to 13-desoxypaxilline by paxP, the latter being then converted to paxilline by paxQ (PubMed:23949005). Finally paxilline can be mono- and di-prenylated by paxD (PubMed:23949005). dimethylallyl diphosphate + isopentenyl diphosphate = (2E)-geranyl diphosphate + diphosphate (2E)-geranyl diphosphate + isopentenyl diphosphate = (2E,6E)-farnesyl diphosphate + diphosphate (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis. Impairs the production of paxilline (PubMed:11169115, PubMed:23949005). Belongs to the FPP/GGPP synthase family.