Converts the inactive gibberellin (GA) precursors GA9 and GA20 in the bioactives gibberellins GA4 and GA1. Involved in the production of bioactive GA for reproductive development. 2-oxoglutarate + gibberellin A20 + O2 = CO2 + gibberellin A1 + succinate Plant hormone biosynthesis; gibberellin biosynthesis. Expressed in flower clusters and siliques. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA3OX subfamily. Extended C-terminus.