During a daily talk show he hosts on WABC radio in New York, Giuliani claimed the investigation is politically motivated and said Atlanta is "well known for corruption," without elaborating.
In an interview with Reuters, Giuliani's lawyer Robert Costello said Giuliani had no intention of answering any questions about Trump that would violate attorney-client privilege.
Giuliani - who served as Trump's personal lawyer and spearheaded his efforts to overturn the 2020 election - appeared before state lawmakers in December 2020, echoing false conspiracy theories about stolen ballots and urging them not to certify Democratic President Joe Biden's victory.
Trump has falsely claimed that he won Georgia, a battleground state that was key to Biden's victory.
In a recorded phone call in January 2021, Trump urged the state's top election official, Georgia Secretary of State Brad Raffensperger, to "find" enough votes to alter the outcome.
The Fulton County criminal probe, one of several investigations into Trump's attempt to invalidate the 2020 election, has ensnared multiple members of Trump's inner circle of advisers.
Also Monday, a federal judge ruled that U.S. Senator Lindsey Graham, a close Trump ally, must testify before the grand jury about phone calls he also made to Raffensperger.
Lawyers for Graham, according to the New York Times, have said that he was informed by prosecutors that he is a witness, not a target.
