New boss Paolo Di Canio claimed his players were "not the fittest team in the world" following Sunday's 2-1 defeat at Chelsea.
But Gardner told the Northern Echo: "We work hard every day, no matter what manager comes in, we're professional footballers.
"I don't know where this has come from, that Paolo Di Canio will work us harder, because every club you're at, there's only one place you'll be found out, and that is on the pitch.
"There's been people talking and chatting about this, but it's a load of rubbish basically. The gaffer's come in and it's the same.
"Every football club has to work hard. We were working hard, we have to if we want to stay in the Premier League. But the gaffer's been brilliant with all of us since he arrived."