On Day 5 of Manafort's trial for tax and fraud charges, Gates testified they worked together to falsify Manafort's tax returns by knowingly failing to report 15 foreign bank accounts, which they knew was illegal. He pleaded guilty earlier this year to conspiracy and other charges as part of an agreement to cooperate with federal prosecutors.
Renato Mariotti, a former federal prosecutor, said, however, that Gates' testimony will be key for the jury and their deliberations.
After Manafort left the campaign in August 2016 amid swirling questions about his Ukraine work, Gates stayed on the campaign and later helped found pro-Trump advocacy group, before he was ultimately forced out, too, as questions mounted about Manafort.
Manafort's attorneys have signaled they will seek to blame Gates and have accused him of embezzling millions of dollars from Manafort.
Throughout his testimony, Mr Gates largely avoided making eye contact with Mr Manafort, who appeared to be watching him.
Gates testified that he and Manafort had 15 foreign accounts they did not report to the USA government, and knew that was illegal.
Those admissions are likely to become a focal point for Manafort's defence team when he is cross examined on Tuesday. Oh, and he says he did it all "at the direction of Mr. Manafort".
The criminal case has nothing to do with either man's work for the Trump campaign and there's been no discussion during the trial about whether the Trump campaign co-ordinated with Russian Federation - the central question Mueller's team has tried to answer.
The trial marks the first test of the special counsel Robert Mueller's investigation into possible collusion between the president's election campaign and Russian agents.
Andres said he was obligated to show the jury why Manafort was getting tens of millions of dollars in payments to prove it was income.
Gates also copped to inflating Manafort's income in doctored financial statements he provided to banks where Manafort was applying for loans once DMP's revenue stream from Ukraine dried up in 2014.
The government called Gates, its star witness, to the stand Monday, just five days into what prosecutors originally expected would be a three-week trial.
But Tuesday, Manafort did not stare Gates down. In questioning Laporta on Monday, a prosecutor asked the accountant about a $10 million loan purportedly received by Manafort from Russian businessman Oleg Deripaska in 2006. Previous evidence has indicated that Peranova, a company Gates said Manafort controlled, never loaned Manafort money.
As a sober Gates recited his testimony, Manafort reacted about the way you'd expect. Gates joined Trump's campaign as deputy campaign manager in June 2016 and continued in other roles after Manafort was ousted.
Since the trial started before US District Judge T.S. Ellis last Tuesday, Manafort's lawyers have kept their cross-examinations brief and at times refrained from attempting to rebut damaging testimony in detail.
And if Manafort did indeed open those accounts, that inherently means he knew they existed - and that he failed to report them to his tax preparers.
Manafort, 69, meanwhile, is facing 18 felony charges that could carry a maximum of 30 years in prison.
Laporta said she had grown to distrust the information Gates was providing her, though she didn't know about the embezzlement.