You are correct in that Gwangju is the watershed moment of anti-Americanism in South Korea. It indeed confirmed that US is not on the side of democracy in South Korea, as they legitimized Chun regime and condoned the massacre.
On the other hand, while US formally held (still holds) "the ultimate authority over the South Korean military", what it meant in practice is questionable. US troops "occupying the country" is not really the correct description. I'd say "assented when consulted" rather than "explicitly gave the orders". Many description is possible, but I think the best description of what US did about Gwangju is "nothing".
It is certainly arguable that US should have done something instead of nothing, but unfortunately I don't think that is the normal standard. Also Korean war should be evaluated separately from Gwangju.
On the other hand, while US formally held (still holds) "the ultimate authority over the South Korean military", what it meant in practice is questionable. US troops "occupying the country" is not really the correct description. I'd say "assented when consulted" rather than "explicitly gave the orders". Many description is possible, but I think the best description of what US did about Gwangju is "nothing".
It is certainly arguable that US should have done something instead of nothing, but unfortunately I don't think that is the normal standard. Also Korean war should be evaluated separately from Gwangju.