Gut-Eater is a slur used to describe Native americans in a derogatory way. They eat stomach lining (tripe) of deer, buffalo, etc. Also after the newly-arrived whites killed all the buffalo they would take every thing but the guts. So all the plains Indians had to eat was guts.
The word Gut-Eater is a racial slur that has been used historically to degrade and dehumanize native americans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to native americans. If you are unsure about whether or not it is appropriate to use the word Gut-Eater, it is best to err on the side of caution and avoid using it in any situation.