We shall have to disagree with what we mean by a "war poem". To me, it is a poem which makes you think about the purpose and reality of war itself. Some poems glorify it, some do not. A poem set against a background of war is not necessarily a war poem.

In the case of "Gunga Din", warfare is simply what has brought the two men together, and the battle is simply the situation which made the speaker realise that Gunga Din was the better man. The nature of that warfare and the reasons behind it are irrelevant to the story. The battle might have been fought for good reasons or for bad, it makes no difference.