Catalyzes the conversion of geranylgeranyl diphosphate to the gibberellin precursor ent-copalyl diphosphate. (2E,6E,10E)-geranylgeranyl diphosphate = ent-copalyl diphosphate Inhibited by high concentrations of magnesium. Plant hormone biosynthesis; gibberellin biosynthesis. Expressed in roots, leaves, flowers and also in siliques. The Asp-Xaa-Asp-Asp (DXDD) motif is important for the catalytic activity through binding to Mg(2+). The N-terminus is blocked. Belongs to the terpene synthase family. Tpsc subfamily.