Catalyzes the conversion of geranylgeranyl diphosphate to the gibberellin precursor ent-kaurene diphosphate in a two step process. ent-copalyl diphosphate = diphosphate + ent-kaur-16-ene (2E,6E,10E)-geranylgeranyl diphosphate = ent-copalyl diphosphate Binds 3 Mg(2+) ions per subunit. Plant hormone biosynthesis; gibberellin biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.