Germany used it most often, then France, followed by Britain

French were first to use gas in the war; In August 1914 they used tear gas grenades containg xylyl bromide on The Germans.

Chlorine was used by Germany in the second battle of Ypres. In THE LATE AFTERNOON OF April 22, 1915, members of a special unit of the German Army opened the valves on more than 6000 steel cylinders arrayed in trenches along their defensive perimeter at Ypres, Belgium. Within 10 minutes, 160 tons of chlorine gas drifted over the opposing French trenches, engulfing all those downwind.

Germans used mustard gas against Russia in September 1917 during the battle of Riga. This damaged internal organs, blistered the internal and external body, and those who weren't
injured were left blind.