"The gurgling starts again - but how slowly a man dies!"

When somebody nears death, saliva may build up in the throat as they lose the ability or the reflex to swallow.  This causes a "death rattle", the gurgling that Paul describes.  Many men were left to die slowly in no-man's-land, and such gurgling was audible to soldiers in the trenches.  Many soldiers went insane listening to the groans of dying men, and it was common practice for soldiers to try to shoot men in their death throes.