With the Black Cats trailing, Gardner was shown a straight red card for a high challenge on Stoke City's Charlie Adam, before John O'Shea scored a late equaliser for the hosts at the Stadium of Light to secure a 1-1 draw.
Afterwards the Italian lamented that fact that he would be without the suspended midfielder for the rest of the season.
"Craig is out but we can't focus too much on that and we will try not to make any further mistakes," Di Canio told Sky Sports News.
"I do love the passion, though, of the players."
Sunderland are now three points clear of the relegation zone having played a game more than 18th-placed Wigan Athletic.