The speaker kisses a gunshot wound, hoping the act of love might change it into something bigger and brighter: "light / or some great fire." It is a wish more than a plan. Kissing is tender and small, and the speaker reaches for it because nothing else is within reach. The wound is "hers," and the poem gives us no more than that, so we are left to imagine who she is and how she was hurt.
The last two lines undercut the hope. The gun "could not / be got out," a phrase that is oddly vague. It might mean a bullet that can't be removed, or something deeper that can't be undone. Either way, love has limits here. The line break after "could not" makes the failure land with a small jolt.
The mood is quiet, aching, and helpless. The speaker's devotion is real, but so is the sense that tenderness cannot erase violence.