I’m not really sure how to answer this. It seems that the term gachi covers a few different concepts, and some are “probably NSFW” and others are, “quite likely not NSFW, unless there’s something else” there. I’m seeing content which goes from gambling, gay porn, muscular (men?), so… it can be tricky to know what you are meaning. I’ve not really even dug that deeply into it, just enough to see that the meaning can be several different things.
I would say, tagging it NSFW if you are concerned that it should be, is not really a problem. And I’m not sure that if you’re willing to lean toward that, but feel it probably isn’t, not tagging it, and finding out later that maybe it should have been is a big problem. Personally, I would lean more toward tagging it as NSFW if that might be where it belongs. But that’s because I have seen people get up in arms that “kids are learning these terrible things in school these days” and the “terrible things” are often informed content about “if you’re going to engage in X activity, do so safely.”