P.S. trying to get a quote for that, I found this, which looks interesting: https://www.thedailybeast.com/inside-gore-vidals-cliffside-p...
They do manage to keep his commentary on Capote's death ("A good career move").
He served in the U.S. Army in WWII and was a member of Franklin Roosevelt's honor guard...so that probably was a pretty "deep cut" as an insult for someone with his military background.
Again doesn't make it right, but probably safe to say for him it was not the same as calling someone that from r/The_Donald in 2018.
During the debate, after the infamous “queer” remark, Buckley said that he “served in the infantry during WWII,” implying that he had fought in battle, which he never had.
(for convenience: https://en.wikipedia.org/wiki/Immanentize_the_eschaton )