I've read - somewhere, years ago, can't find it now - that the nickname Gunga Din was a racist one, being a rude name for a chickpea based on the arse shape of the legume. So Kipling was using a contemptuous name for a servant looked down on by all the little twerps of Anglo-Indians as a counterpoint to the regimental bhisti's heroism.
Is it racist? Yes.

"An’ for all ’is dirty ’ide
"’E was white, clear white, inside
"When ’e went to tend the wounded under fire!"