The speaker of the poem describes the appearance of a fellow soldier who does not secure his gas mask over his face before inhaling some of the poison gas. By October of 1917, mustard gas, which is quite deadly, was being used as a chemical weapon during World War I. It is also possible that the speaker could be describing chlorine gas, which is murky green in appearance (and he refers to the air around them as a "green sea"). Mustard gas reacts with the water in our respiratory systems, creating hydrochloric acid, which burns the throat and lungs and creates swelling that can block the airways. Chlorine gas also causes serious lung damage and can cause the lungs to fill it with fluid, leading to death.
The speaker has described watching his fellow soldier inhale poison gas as "drowning," and he keeps reliving the sight of the doomed man "plung[ing] at [him], guttering, choking, drowning." This man's lungs have been irrevocably damaged, burned by the gas, and so it probably feels very much, to him, as though he is drowning, because he cannot take in enough air. The word guttering likely describes the sweat and tears running down the man's face (just as water pools in the gutter). The soldier is literally choking on and drowning in the fluid that is collecting in his lungs. These words also go with the narrator's description of the "green sea" that seems to surround him.
The speaker of the poem describes the fellow soldier who inhales poison gas as “stumbling” around, yelling, and “flound’ring” like a man on fire or as one who is “drowning” and gasps for air. He reports that he has frequent, vivid dreams in which this doomed soldier “plunges at [him], guttering, choking, drowning.” Given this description, it sounds as though the speaker has a clear memory of the poor man’s movements immediately after he has inhaled the poison that will proceed to kill him, slowly and painfully.
People are typically not described as performing the action of “guttering,” though the intransitive verb “to gutter” can refer to something that either flows or inclines downward; for example, a liquid could gutter, or flow, down a hill, or a candle flame can gutter in a draft of wind. Therefore, one can interpret this word choice of “guttering” as the man stumbling toward the speaker or even dropping to his knees in front of the speaker. The man seems to be “choking,” in all likelihood, because he is inhaling something caustic and poisonous, and so he likely would be literally choking on whatever he has inhaled. Finally, this doomed man appears to be “drowning,” though he is not literally drowning. He is, perhaps, flailing his arms and gasping for breath as a drowning man might do. In short, these three present participles are verbs and also function like adjectives, describing the behavior of the doomed soldier after he inhaled the gas that will kill him.