An investigation published by Britain's Telegraph newspaper, using journalists pretending to be making a film, approached Mitchell and an agent linked to Gatlin, Robert Wagner.
The Telegraph claims Wagner offered to "supply and administer testosterone and human growth hormone for an actor training for a film," to reporters for a fee of $350,000.
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.
The paper alleges Mitchell said athletes could dope undetected by tests and that Wagner cited Mitchell and Gatlin as being involved in doping.
Gatlin responded by dismissing Mitchell and issuing a statement.
"I am not using and have not used PED's (performance enhancing drugs)," he said. "I was shocked and surprised to learn that my coach would have anything to do with even the appearance of these current accusations.
"I fired him as soon as I found out about this. All legal options are on the table as I will not allow others to lie about me like this."
Gatlin was first suspended in 2001 over use of an attention deficit disorder medication and then again, for eventually four years, in 2006 after testing positive for testosterone.
In between he won Olympic gold over 100m in 2004 and in his second return to competition, finished second to Usain Bolt in the 2016 Rio de Janeiro Games.
But Gatlin, by now aged 35, defeated the great Bolt to win this year's 100m at the world championships in London.
The Telegraph quoted Sebastian Coe, president of the athletics governing body IAAF, as saying: "These allegations are extremely serious and I know the independent Athletics Integrity Unit will investigate in accordance with its mandate."
Mitchell himself was a former athlete, winning Olympic and worlds bronze over 100m and gold with the United States 4x100m relay team at the 1992 Games and worlds in 1991 and 1993.