We would have loved to seen that!
Gerard went on stage with "mistress of ceremonies" cabaret star Lady Rizo aka Amelia Zirin-Brown who called him "Mistress Butler."
But the best part was his "outrageous version of Mustang Sally" with John Mayer.
An eyewitness at the party said:
"Gerry also sang Oasis hit Wonderwall with Lady Rizo and Mayer played guitar. They then followed it up with Mustang Sally. He had a pretty good voice — it wasn't what you'd expect from the guy from 300. It was epic. He was also in the deejay booth a lot and got the deejay to play a lot of rock, '80s and funk.
Gerry announced to the crowd that Mayer was going to sing for them, but John replied he just wanted to play guitar while Lady Rizo sang. So he had the crowd cheer to hear Mayer perform. Mayer then sang his hit Who Says."
Sounds like an amazing party! Happy 41st birthday, Gerry!
[Image via WENN.]