The Solihull-born midfielder says his team-mates cannot "keep talking" about improving after suffering bad results and must show they can do better out on the pitch
Martin O'Neill's side sit just one point above the Premier League relegation zone having won only two games in 14 so far this term.
And the 26-year-old, who netted in the 2-1 defeat to Norwich on Saturday, has called for his side to build on their improved second-half performance at Carrow Road.
"We can't afford to keep talking after games about what we have to do,” Gardner told the Daily Star.
"We have to start going out there and doing it. There's no excuses for us anymore. We can't just keep saying things will be fine - we have to make them fine.
"We know we can do it, our second-half performance against Norwich showed that. We know if we play like that, then we'll start winning."
The former Birmingham midfielder is now hoping the Black Cats can do better when they host reigning European champions Chelsea at the weekend.
"We are desperate for a win. We need to start pushing on," he added.
"Chelsea will be tough but if we can take that second-half performance into Saturday's game, we can win.
"We're slowly starting to find our form. The problem is we need to find it quickly."