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I think the following is true, but feel free to correct me if I'm wrong.
The word "gay" was being used to describe men who would later be called homosexuals by the mid-19th century. I think I remember reading this in conjunction with something about Walt Whitman.
The word "homosexual" was coined by Victorian-era doctors, influenced by Freud and all that. It was created as part of the movement to medicalize human behaviors that were previously seen as moral failings; if they were diseases, then they might be curable.
The point - "gay", in that sense, preceded "homosexual" by at least a few decades.