Gay was slang, at least in the 20th and 21st centuries on Earth, for someone whose sexual preference was for someone of the same gender. It was essentially synonymous with queer. (PROSE: Damaged Goods) In this sense, it was not necessarily a pejorative, but merely a statement of sexual orientation.
However, it was also a clear insult, meaning weak or lame. Rose Tyler, for instance, once claimed that the Ninth Doctor was "so gay" for complaining about the pain of being slapped in the face by her mother. (TV: Aliens of London)