Manafort, who served briefly as Trump's campaign manager in 2016, has pleaded not guilty.
"He had these two guys running his campaign!"
Gates also admitted, however, that the same tricks he used to help doctor and falsify records for Manafort he also deployed for his own personal gain in order to covertly wire funds out of Manafort's offshore accounts to line his own pockets.
Manafort's lawyers are determined to impugn the credibility of Gates, who pleaded guilty in Mueller's investigation and agreed to cooperate with investigators by testifying in the financial fraud trial.
"After all the lies you've told and the fraud you've committed, you expect this jury to believe you?"
Gates conceded it was "in excess of 50 to 100 years", but said any eventual sentence - including avoiding jail by receiving probation - would be up to a judge.
On Wednesday, Downing sought to counter earlier testimony that Manafort had encouraged Gates to deceive authorities by getting Gates to acknowledge that Manafort told him to be truthful about offshore shell companies and bank accounts during a 2014 interview with the Federal Bureau of Investigation. Mr. Manafort had the same path; I'm here.
A U.S. Internal Revenue Service agent testified on Wednesday that former Trump campaign chairman Paul Manafort, who is on trial on tax and bank fraud charges, had $16.5 million in unreported taxable business income between 2010 and 2014. Gates also had his name removed from some of the legal documents.
Earlier on Tuesday, the prosecution had continued to question Gates, who described how millions of dollars were allegedly funnelled from entities controlled by Ukrainian businessmen into Manafort's shell companies in Cyprus.
Special counsel Robert Mueller's team, which is investigating Russian interference in the 2016 election, says Gates was heavily involved in helping hide millions overseas.
Manafort's defense team did its darndest to sully Gates as a liar, a cheat, a thief and an adulterer for eight hours inside Judge T.S. Ellis' Alexandria courtroom over the past three days. The defense moved on.
Mueller, a former Federal Bureau of Investigation director, has indicted more than 30 people - including 26 Russians - in connection with his probe into whether members of Trump's campaign colluded with Russia to help get the real estate tycoon elected.
The prosecution produced an undated email exchange between Mr Manafort and Mr Gates which outlines the amount of tax the campaign chief might have to pay. He told jurors he embezzled from Manafort by filing false expense reports. That, Downing said, would invalidate his plea deal.
Meanwhile, Manafort's former accountant, Cindy Laporta, was sacked from her firm on Tuesday after testifying last week that she helped falsify his tax records, The Hill reported.
Gates' testified that he and Manafort knew they were committing crimes for years.
Gates acknowledged that he did for two months and that he took first-class flights and stayed in "fancy hotels" around Europe, but said the money came from bonuses rather than illicit gains.
Paul Manafort recommended Stephen Calk, a Chicago banker involved in approving a loan at the center of Manafort's financial fraud trial, as secretary of the Army, according to testimony Tuesday from star witnessRick Gates.
Manafort is the first person to be tried on charges brought by special counsel Robert Mueller's investigation into Russian interference in the 2016 US presidential election.
The case has little to do with either man's work for the Trump campaign and there's been no discussion during the trial about whether the Trump election effort coordinated with Russian Federation - the central question Mueller's team has tried to answer.
Trump has recently sought to distance himself from Manafort, tweeting that the former top campaign official "worked for me for a very short time".
The firm said it was unaware of any wrongdoing by Laporta before she testified and has never had an employee disciplined by a government agency in the firm's 35-year history. Gates said the man schooled them on how to shield their operations from scrutiny by tax collectors.
Prosecutor Greg Andres pointed out he had created a "loan forgiveness letter between Mr. Manafort and Mr. Manafort".
He also admitted to embezzling money from other employers in the past. A bookkeeper said she helped disguise foreign income as a loan to reduce Manafort's tax burden.