Evidence of "gay" being used as a synonym for "bad"?

My mom doesn't believe me when I describe the "that's gay" slur. I told her about the Think Before You Speak PSA, and she says that must have been made by a minority of gay people who got offended by a few homophobes and overreacted, and that any contemporary usage of the term is in response to the PSA. Is there any evidence of the term being used prior to the PSA that I could show her?