Where does it say that gay people co-opted the word “gay” for themselves? It could have been put upon them, along with words like “fag” etc. Just saying, I don’t know that they actually hijacked the word for themselves. The rainbow as a symbol of gay pride on the other hand....
And the word “gay” is still very much a derogatory term in many respects. “That’s so gay” is common derogatory expression used, especially by younger people.