Methylates the carboxyl group of several gibberellins (GAs). Substrate preference is GA4 > GA34 > GA9 > GA3 > GA1 > GA51 > GA20. No activity with diterpenes abietic acid and ent-kaurenoic acid. gibberellin A4 + S-adenosyl-L-methionine = O-methyl gibberellin A4 + S-adenosyl-L-homocysteine Binds 1 Mg(2+) ion per subunit. Down-regulated by Zn(2+), Cu(2+) and Fe(3+). No effect of K(+), NH(4+), Na(+), Ca(2+), Mg(2+), Mn(2+) and Fe(2+). kcat is 0.0015 sec(-1) for GA4. kcat is 0.0018 sec(-1) for GA9. Optimum pH is 8.0. Expressed in siliques 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. Up-regulated by alamethicin, but not by herbivory. No visible phenotype, even in gamt1 and gamt2 double mutants. Overexpression of GAMT2 results in dwarf phenotype. Belongs to the methyltransferase superfamily. Type-7 methyltransferase family. SABATH subfamily. Truncated N-terminus.