In poetry, imagery can be defined as a form of description that appeals strongly to the reader's senses. In Hopkins's “I wake and feel the fell of dark, not day,” the senses of the reader are stirred by a variety of images associated with the body.
In one such example, in the very last line of the poem, Hopkins refers to “sweating selves.” This conjures up an image of bodily sickness caused by a particularly nasty fever.
The speaker's bodily sickness appears to be a manifestation of a deep spiritual crisis. Isolated from other human beings, the speaker feels so alone that he appears convinced that he's been abandoned by God, and this understandably makes him feel incredibly depressed. All his laments and countless cries to the Almighty are like so many letters that have been sent away but never opened.
Hopkins uses bodily ailments to express just how badly he feels. He is “gall,” he is “heartburn.” What he believes that God has decreed for him—this deep despair generated by a profound spiritual crisis— leaves a bitter taste in his mouth, just like a particularly bad case of indigestion.
As Hopkins's soul is so deeply mired in torment, its relationship to his body has become dangerously unbalanced, to the extent that the body appears to be gaining primacy. For a devout Christian like Hopkins, this is not how things ought to be. On the contrary, it's the soul that must be in charge, not the body.
But because Hopkins's soul has been so damaged, so tormented by a seemingly insoluble spiritual crisis, the language of the spirit, the language of the soul, is no longer adequate to express his feelings. And so the body and the language associated with it—“gall,” “heartburn,” “sweating selves,” and so on—come to the fore, a clear indication that something is seriously wrong.