Robbie drops in on Gary on stage
Robbie, who has just announced a European arena tour, performed singles Eight Letter and his hit Candy towards the end of the set, The Sun reported.
The newspapers said that Gary's gig almost didn't happen, because the water had been turned off throughout the building.
Just a few minutes before going on stage, the X Factor judge said: "At the moment the show's off - there's no water in the building and because of health and safety you can't start a gig without it. Hopefully it comes on soon because at the moment I'm not going to be playing tonight."
But the show did go on when the water was turned on, and Gary went on to perform his hits in front of the crowd.