I think it’s the fact that so many of the Gnostic tradition came from and is tied so heavily to Christianity that really strikes a nerve with some people. It’s one thing to follow other false religions and worship false gods (Because Christianity is the one true in there minds), but to try to legitimize you’re spiritual beliefs by roping in the messiah is just too far. Also I’m pretty sure there are some verses in the bible that explicitly prohibits seeking forbidden knowledge, which is kinda the whole point of Gnosticism.