In Chapter 4 of All Quiet on the Western Front, the men fall into several attacks, and by the time gas is used, the men are in a graveyard. All along, Kat has been having premonitions about oncoming attacks, and he does so again. Paul has collided with a dead body from one of the bombed graves, and then Kat shakes him and yells that gas is being used so that Paul can put on his mask. The men then signal by mouth to each other that gas is being used in the attack. Eventually, a bell sounds amid the noise of shellfire, indicating the use of gas so that everyone can put on his mask:
A bell sounds between the explosions, gongs, and metal clappers warning everyone--Gas--Gas--Gas.
The gas from the shells covers the ground, forcing the men who have gone underground to come back to the surface. Paul notes that the first few minutes with the mask could be the difference between life and death, so the bell indicating the attack is crucial.