Gordon was on Wearside yesterday to complete a medical and discussed terms today.
Reports claimed the Scot, 24, was due to have talks with Aston Villa this week too - with a view to choosing his career path on Friday. However, this has turned out not to be the case - although Villa did express an interest.
The transfer puts into doubt the future of Marton Fulop who spoke of his ambition to be Hungarian number one back in June. Click here to read this story.
Meanwhile, Sunderland are competing with two other Premier League clubs for the signature of Egyptian striker Mido. The Spurs star is available for a fee of around £6m and is being chased by the Black Cats, Birmingham and another un-named club.
Mido was poised to complete a move to the Blues last month but talks broke down. However, the Egyptian has revealed that Steve Bruce's side are still very-much in the race for his signature.