Show-stopper: Lady GaGa dressed - or rather under-dressed - in a black leotard and killer heels at the Carnegie Hall bash for Bono's pal Gavin Friday

Lady GaGa helped out new pal Bono Sunday night with an extra special gift for Gavin Friday's 50th birthday bash.

Dressed in just a sheer leotard with two blacked-out crosses over her breasts, Lady GaGa teetered over to Friday on towering stilettos to present the birthday boy with his birthday cake.

Lady GaGa said Bono had called her earlier that week to ask her to play at the star-studded gig in Carnegie Hall (Is there a private 411 for megastars do you think?)

"He was like 'oh it's just this little downtown thing'," she said, "And I'm thinking to myself as I arrived here [at Carnegie Hall] tonight: 'This isn't 'downtown' to me!'

"But I guess when you regularly play to 90,000 fans it might feel like 'downtown'."

Lady GaGa joined Scarlett Johansson, Courtney Love, Christy Turlington, Shane MacGowan and the boys from U2 to help celebrate Friday's birthday at Carnegie Hall.