Catalyzes the conversion of geranylgeranyl diphosphate (GGPP) to the gibberellin precursor ent-copalyl diphosphate (CPP). (2E,6E,10E)-geranylgeranyl diphosphate = ent-copalyl diphosphate Plant hormone biosynthesis; gibberellin biosynthesis. The Asp-Xaa-Asp-Asp (DXDD) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.