Key oxidase enzyme in the biosynthesis of gibberellin that catalyzes the conversion of GA12 and GA53 to GA9 and GA20 respectively, via a three-step oxidation at C-20 of the GA skeleton. 2 2-oxoglutarate + gibberellin A12 + H(+) + 3 O2 = 3 CO2 + gibberellin A9 + 2 H2O + 2 succinate 2 2-oxoglutarate + gibberellin A53 + H(+) + 3 O2 = 3 CO2 + gibberellin A20 + 2 H2O + 2 succinate Binds 1 Fe(2+) ion per subunit. Plant hormone biosynthesis; gibberellin biosynthesis. Expressed in roots. Detected in leaves, inflorescences and siliques, but not in stems and dry seeds. Not controlled by the level of physiologically active gibberellin or by auxin. Up-regulated by paclobutrazol. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA20OX subfamily.