Gas was first used as a weapon at the Second Battle of Ypres in April 1915, at this point soldiers had no protection. Three main gases were used on the front:

Chlorine gas: the gas stripped away the linings of the lungs, causing immediate choking and eventual victims drowning from water produced in their own lungs.
Phosgene gas: 18 times more deadly than chlorine; 48 hours until death. In this 48 hours, victims experience spasms and vomiting; the lungs fill up with yellow liquid and the victim drowns.
Mustard gas: first utilised in 1917 but not that deadly as 2% of victims died. The gas attacks the surface of the skin causing intense burning.

Gas attacks clogged up medical treatment areas with doctors having to give sufferers oxygen and wash skin to remove traces of poison gas.