Birmingham's relegation from the Barclays Premier League made his departure almost inevitable, and the Black Cats were quick to make their interest known and then persuade Gardner that his future lay at the Stadium of Light.
Gardner told SAFC TV: "It's unbelievable, to be honest. As soon as I found out Sunderland were interested, it was a no-brainer.
"Obviously, I have heard so much about Steve Bruce and his backroom staff and how good a chairman Niall Quinn is.
"It's such a massive club and their ambitions for this season meet my ambitions, and it was too good to turn down."
The midfielder, who scored 10 goals in the last campaign, is confident he can bring the same potency to Wearside, where he is setting his sights high after hearing Bruce's recruitment plans, which have already landed him five new signings.
He said: "It means that the club wants to go places. Obviously, we want to finish as high up the table as we can - we want to be in Europe next year and it's a great task to go and chase.
"It's a good goal for us and we can go and do it, and I am sure we will."