Gatlin most recently took gold at the IAFF World Championships, beating Usain Bolt in Bolt's final race.
Wagner also denied being involved in doping, but said he "played along" in the sting because he knew what was going on.
He added: "I made up the comments to impress them, ..."
The Telegraph report published Monday said the undercover reporters traveled to Gatlin's Florida training camp and presented that they were making a film about athletics, and that they needed help training a male actor so he could get in shape.
And the paper also alleges former Olympic gold medallist Mitchell, told reporters that athletes are able to get away with doping because the drugs they use can not be detected by tests.
"It was just big talk - I did not actually source or supply the substances the reporters asked for but stupidly claimed I could", the US-based Austrian said.
A statement from Gatlin said: 'I am not using and have not used performance-enhancing drugs.More news: Looks like Florida State should not be bowl eligible this year
In a statement to the Telegraph, Mitchell said: "I never suggested in any way that any of my current athletes used any banned substances or that I was familiar with training any of my current athletes with those substances". "I fired him as soon as I found about this", Gatlin said.
"All legal options are on the table as I will not allow others to lie about me like this", he stated.
The American has a history of cheating - he served a one-year ban in 2001 and a four-year suspension after testing positive for performance-enhancing drugsagain in 2006.
"These allegations are very serious and strike at the heart of the integrity of athletics", said Brett Clothier, the head of the Athletics Integrity Unit, an independent entity whose mission is to ensure clean sports competition.
And the matter is now the subject of an investigation by the United States Anti-Doping Agency (USADA) and the Athletics Integrity Unit (AIU), according to the Telegraph and the Press Association. "They will next hear from my lawyer".
Athletics anti-doping officials have begun an investigation into what the president of the sport's governing body called "serious allegations" about Dennis Mitchell and Robert Wagner.
"Under the IAAF Rules, all athlete support personnel - agents, coaches etc - are bound by both the IAAF anti-doping code and IAAF integrity code of conduct".