The Geneva Protocol was an international treaty passed by basically everyone in the later 1920's (different people signed it in different years; France in 1926 and Germany in 1929 for instance). You can read the full text of it here, it's pretty short:

Whereas the use in war of asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices, has been justly condemned by the general opinion of the civilized world;

and Whereas the prohibition of such use has been declared in Treaties to which the majority of Powers of the world are Parties; and

To the end that this prohibition shall be universally accepted as a part of International Law, binding alike the conscience and the practice of nations;

In short they banned it internationally. That didn't stop some though; Japan is recorded having used chemical warfare against the Chinese for instance in WWII. The Americans, British, and Germans had massive stockpiles of chemical weapons as well. The main reason they didn't use them, frankly, is the same principle of MAD for nuclear weapons. A little more technically if Germany were to use chemical weapons against the Soviets or British or whomever the British/Americans, with total air dominance over the Germans, would just level them with it.

Ultimately gas warfare did very little in the way of actually winning battles in WWI. There were less than 100,000 total fatalities caused by chemical agents, less than 0.1% of total deaths. It did a damned good job at maiming people, about 1,000,000 people would be considered non-fatal casualties due to chemical agents in WWI...2.5% of the total and ~5% of the military casualties. Its only purpose was terror. For the Germans employing this weapon, especially on civilians, would give the Allies every justification to carpet their cities in gas in retaliation. It would turn the battlefield unnecessarily more brutal while only, in the long run, make things even worse for their situation.