When the company enters the bar for Lorelai’s bachelorette party, a drag queen dressed as Judy Garland is lip-synching on stage to this song.
Get Happy is a song written by Harold Arlen, with lyrics by Ted Koehler; the lyrics mimic gospel and evangelical songs. It was first performed by Ruth Etting in the 1930 Broadway show Nine-Fifteen Revue, and although the musical was a disastrous flop, Get Happy was successful and recorded several times, including by both Benny Goodman and Charlie Parker.
Judy Garland sang the song in the 1950 musical film Summer Stock, her last movie for MGM. In a let’s-put-on-a-show plotline, Garland performs the number wearing a tuxedo jacket, fedora, and stockings and high heels in the movie’s most iconic scene, often copied by other singers and dancers.
Judy Garland had been a gay icon since The Wizard of Oz became popular TV viewing, so having this song playing as they enter is a stereotypical way to signal it’s a gay club.