Gardner believes the fact the 30-year-old has played for manager Martin O'Neill previously will help him settle straight in at the Stadium of Light.
The midfielder told the Birmingham Mail: "Carlos knows what the gaffer wants, which is crucial, and his drive and enthusiasm for the game are unbelievable.
"I was with Carlos for two years at Villa.
"He is a top lad - solid, strong and he is a really nice lad to have around the place.
"All the lads have welcomed him and he is enjoying it. "He has a lot of experience from playing in Spain and at Villa.
"He always talks - although you might not be able to understand him at times! - and he makes sure everyone stays on their toes."