Catalysis of the reaction: 2 2-oxoglutarate + gibberellin A12 (GA12) + H+ + 3 O2 = 3 CO2 + gibberellin A9 (GA9) + 2 H2O + 2 succinate. This reaction results in the oxidation of C-20 gibberellins to form the corresponding C-19 lactones, via a three-step oxidation at C-20 of the GA skeleton. Also converts GA53 to GA20. GA25 is also formed as a minor product.
gibberellin 20-oxidase activity