O'Neill has come in for some criticism from fans after a frustrating season which has left the Black Cats only four points above the Premier League relegation zone with eight games remaining.
However Gardner, who also played under O'Neill at Aston Villa, has backed the Ulsterman to steer his side to safety.
Gardner told The Journal: "I have worked with the manager for seven or eight years. He is one of the best. He is unbelievable.
"He turns things around at times like this. He is one of the best and we will be fine. We just need to dig deep and we will.
"Martin O'Neill is Martin O'Neill. He knows what to do. He has been in the game longer than I have been alive.
"He has been through every single situation possible. He knows what to do. We have the best possible person in charge. He will get us out of it."