The Scotland international is the club's leading goalscorer so far this campaign, having found the net on 10 occasions.
With that statistic in mind, Gardner has described Fletcher's contribution as "phenomenal".
"To have scored 10 goals at this point in the season is phenomenal," he told the club's official website. "If someone like Rooney, Messi or Van Persie had scored those goals you would never stop hearing about them for months. I wish Fletch would get the same recognition because he's different class."
Fletcher joined the Black Cats last summer from Wolverhampton Wanderers.