Methylates the carboxyl group of several gibberellins (GAs). Substrate preference is GA9 > GA20 > GA3 > GA4 > GA34 > GA51 > GA1 > GA19 > GA12. No activity with diterpenes abietic acid and ent-kaurenoic acid. gibberellin A9 + S-adenosyl-L-methionine = O-methyl gibberellin A9 + S-adenosyl-L-homocysteine Binds 1 Mg(2+) ion per subunit. Up-regulated by K(+) and NH(4+), down-regulated by Zn(2+), Cu(2+), Fe(2+) and Fe(3+). kcat is 0.01 sec(-1) for GA4. kcat is 0.026 sec(-1) for GA9. Optimum pH is 7.5. Expressed in siliques, developing seeds, anthers and germinating seeds. Not detected in leaves, stems, flowers and roots. Expression begins at early stages of silique development, peaks in the second half of this process and decreases after the start of desiccation. No visible phenotype, even in gamt1 and gamt2 double mutants. Overexpression of GAMT1 results in dwarf phenotype. Belongs to the methyltransferase superfamily. Type-7 methyltransferase family. SABATH subfamily.