Sentence:  ...and to take that fire behind the bony bars of the chest and into the tower of the windpipe, in one breath, before you choke on a <WSD>gag</WSD> of air thickened from the last breath of the executed the breathing of hot barrels and blood streaming on concrete,... 
Meaning of the word marked with <WSD> tags:  A mouthful that makes one retch or choke.