The Scots-born star took to the stage with the singer-songwriter at New York restaurant The Darby.
They belted out Oasis hit Wonderwall, as well as a version of Mustang Sally.
And he was joined on stage by the eaterie's 'mistress of ceremonies' - cabaret star Lady Rizo.
She told guests: 'Some of you know him as Gerry, others as Gerard. I know him as Mistress Butler.'
A guest said: '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 DJ booth a lot and got the DJ to play a lot of rock, '80s and funk.'
Guests at the bash included Kate Hudson, Adrien Brody, Bono and Jay-Z, the New York Post reports.