The spiky-haired singer paid a surprise visit to besotted schoolgirl Lisa Dragoonis after she won a competition on his official website.
But Gareth was left standing on the doorstep on Hallowe'en because Lisa's father, Peter, was watching the Celtic v Blackburn match with the volume at full blast.
The distraught schoolgirl realised she had missed out when she found a Polaroid picture in the letter box 10 minutes later - after coming home from maths tuition.
The photo showed the pop star dressed up in a Phantom of the Opera costume and grinning outside the front door of her house in Giffnock, near Glasgow.
There was also a hand-written message saying: "To Lisa, I popped in to say hello but you weren't in! Lots of love, Gareth."
Lisa, 15, said: "I am absolutely gutted. I am his biggest fan and have been since the start of Pop Idol.
"I haven't got over this and I'm not going to speak to my dad for ages."
She added: "The picture is in my room, in the middle of three folders and in a CD case so I don't lose it."
Her red-faced dad, a 45-year-old salesman, said: ""Lisa is daft on Gareth. Her room is full of posters and pictures of him. I'll be in the dog house for ages."
"Lisa mentioned the fact that she had entered this competition to her mother but not to me.
"When the match started I turned the lights off and had the telly blaring so when the doorbell rang I didn't hear it.
"When she came home ten minutes later the air turned blue. She was beside herself.