Agent Ian Elliott had claimed the offer his client rejected recently did not reflect his club's apparent £12million valuation, and suggested he would leave for free after seeing out the remaining two-and-a-half years of his current deal.
Gibson responded today by claiming Elliott's actions may not be in the player's best interests, prompting an angry riposte.
Elliott said: "I am very saddened by the comments made by Steve Gibson in today's papers.
"I strongly deny that I am not acting in the best interests of my client.
"Stewart Downing has been a client of mine for almost 10 years. I would consider him and his family close friends as well as clients.
"I would never act in anything other than his best interests.
"I consider the remarks by Mr Gibson to be defamatory and the matter is now in the hands of my legal advisors.
"I will be making no further comment on this subject."