The Geneva Protocal was signed in 1925 by 16 major powers forbidding their use. Almost all the belligerents in WW2 had stockpiles and were prepared to use them, but mainly as a retaliatory measure if the enemy used them first. Exceptions being Japan who used them in China and there are allegations that Germany used them against Soviet Russia.

You might be interested in some previous threads on this topic:

• The use of poison gas on troops in WW2
  ^([2014 Jul 07 | 12 comments])
  ^/u/vonadler ^(points out the practical problems) ^(involved in using gas and /u/k1990) ^(describes political reasons against it.)

• Poison Gas WWII
  ^([2012 Dec. 05 | 15 comments])
  ^(/u/BeondTheGrave discusses how gas was more suited to WWI warfare on the Western front than it was for WWII warfare.)

The Germans had nerve gas, the Allies didn't. However, due to a ban on publishing scientific papers on organophosphate toxins (of which the German nerve agents: Sarin and Tabun were examples of,) Germany assumed the allies also had nerve agents. Really, organophosphates were banned in the scientific press due to the discovery of DDT, a really useful insecticide that would lower attrition due to disease in the users army.

http://vectorsite.net/twgas_2.html