What does Gas Gas quick boys in Wilfred Owens Dulce Et Decorum Est mean?

This is a poem about the first world war. The poem starts with a description of soldiers marching away from the battlefield, they are incredibly tired and hoping to get some rest.

Gas! Gas! Quick boys! means there is an attack with mustard gas, which was frequently used in that war. So they need to put on the gasmasks. One of the soldiers doesn't succeed in doing this in time and he chokes.