One of the challenging aspects about studying literature from a different time period is the way that vocabulary is used which had meaning then but has now become anachronistic. Often, unless we are given a definition of those words, we need to very carefully read such words in context to work out what they mean and what they might refer to. This, of course, depending on your perspective, is a great opportunity to increase your vocabulary and consider how words and their meaning change over time!
So, when we think of the eyepieces of the gas mask, the gas masks are referred to in the second stanza. Consider what happens: there is a sudden gas attack and the soldiers, in an "ecstasy of fumbling," put on their gas masks. All except one is successful, and the speaker, with his gas mask on, watches him die a terrible death:
Dim, through the misty panes and thick green light,
As under a green sea, I saw him drowning.
Thus we can deduce that the eyepieces of the gas mask are called "panes" as the speaker of the poem has to look through the "panes" to see his companion dying.