The Take That frontman serenaded Jo Chapman with a performance of the group's hit 'Greatest Day' after turning up at her special bash in Maidstone, Kent, south east England, and she was "in shock" when she saw the 45-year-old star, reported Female First.
"When he walked through the door I just thought, 'Oh my God.' I couldn't believe it. I just screamed like a lunatic and ran over to him. I was surprised I was still conscious. It was just unbelievable, I'm still in shock," Chapman said.
Her friend Suzi Cooper was then contacted by a member of Gary's team around three weeks before her birthday bash to arrange his appearance and she managed to keep it a secret from her.
"Some people that saw it online thought it was a wind up and that it was an impersonator, so I had to show them a tweet Gary had sent me afterwards to prove it.
"Never in a million years did I think he'd actually come. The DJ played my favourite Take That song Everything Changes and when that finished I thought, 'Well that's it now.' Then Greatest Day started and I saw him. It's still sinking in that I met him," Chapman said.