Heterodimeric geranyl(geranyl)-diphosphate (GPP) synthase small subunit. The small subunit alone is inactive in vitro while the large subunit GGPPS1 catalyzes mainly the production of geranygeranyl-diphosphate in vitro. Upon association of the two subunits, the product profile changes and the production of gerany-diphosphate is strongly increased. Binds 2 Mg(2+) ions per subunit. Part of a heterodimeric geranyl(geranyl)diphosphate synthase. Interacts with GGPPS1 or GGPPS2, but not with GGPPS9 (PubMed:19482937). Interacts with LIL3.1 and LIL3.2 (PubMed:20823244). Expressed ubiquitously. Belongs to the FPP/GGPP synthase family.