The main location of the poem, where the poor soldiers are walking when they face a gas attack, is never actually specified. However, we can make a good guess. In World War I, the front line was mainly in France, and so as we are told the soldiers are walking back from the front line we can make the assumption that this is occuring in France. Note how this is made clear in the first four lines of the poem:
Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge,
Till on the haunting flares we turned our backs
And towards our distant rest began to trudge.
The "haunting flares" refer to the flares that either side sent up between the two front lines over no man's land to indicate whether any troops were trying to cross. As they are walking towards their "distant rest" we know that they are walking away from danger and towards some kind of supposed shelter.