Actually, the Japanese often used poison gas against the Chinese in WWII.

The Chinese didn't have any to strike back with. The Japanese never used it against the western Allies, who did. One reason that it wasn't used in Europe is that both sides had it. If anybody had thought that the use of gas, lawful or not, would have given them a decisive advantage, they'd have used it.

More than one treaty banned the use of poison gas after WWI, but industrial nations kept right on developing and manufacturing them for decades - just in case.