Louise Newman, 27, couldn't believe her eyes when Barlow walked in to the reception belting out her favourite 'A Million Love Songs' - then sang 'Relight My Fire' as an encore, reported Daily Mirror.
Louise had just had her first dance with new husband Mark Francis, also 27, when Barlow appeared.
"I heard the opening bars to A Million Love Songs and was swaying and singing along when someone called my name. I looked round and there was Gary. It was just magical. I went up and cupped his face in my hands, then he got down on one knee and sang to me," she said.
Barlow had been planning the surprise with maid of honour Emily Alcorn, who had waged a year-long Twitter campaign #getgary2louswedding.
Fans around the world tweeted pictures of themselves holding the hashtag plea.
And when Barlow, 43, spotted Emily's banner at one of his concerts he agreed - as long as it was kept secret.