Gates said that Manafort told him to get in touch with Philip Ayliff, one of Manafort's tax accountants, but Gates said he could only reach Cindy LaPorta, another accountant who received immunity to testify last week.
Gates, who also was an official on Trump's campaign, pleaded guilty in February to lying to investigators and conspiring to defraud the United States and agreed to cooperate in Special Counsel Robert Mueller's ongoing investigation into whether Trump's campaign colluded with Russian Federation.
Gates is cooperating with the investigation as part of a plea bargain with the prosecution, whilst Paul Manafort denies all charges- which include tax fraud; bank fraud; and failing to report foreign bank accounts.
Manafort has a second trial scheduled for September involving allegations that he acted as an unregistered foreign agent for Ukrainian interests and made false statements to the U.S. government. Previous evidence has indicated that Peranova, a company Gates said Manafort controlled, never loaned Manafort money.
On Monday, Gates spoke in short, clipped answers as Manafort rarely broke his gaze from the witness stand, followed that of vendors who detailed Manafort's luxurious spending and financial professionals who told jurors how the defendant hid millions of dollars in offshore accounts.
Gates has been described by witnesses as Manafort's right-hand man who helped run the operations of a multimillion-dollar political consulting business.
Gates - who served on the 2016 Trump presidential campaign with Mr Manafort - told the court he had stolen from other employers, too. Oh, and he says he did it all "at the direction of Mr. Manafort".
Those admissions are likely to become a focal point for Manafort's defence team when he is cross examined on Tuesday. He also admitted to concealing millions of dollars in foreign bank accounts on Manafort's behalf and to falsifying documents to help his former boss obtain millions of dollars more in bank loans.
Mr Mueller is also investigating possible coordination between Trump campaign members and Russian officials in the election campaign, but the charges against Mr Manafort do not address that.More news: Kroenke set to buyout Usmanov in £1.8 billion Arsenal deal
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In opening arguments, Manafort's lawyer defended his client by blaming Gates for the offenses alleged against his client.
But Ms Laporta's testimony raised the stakes for Mr Manafort, legal experts said.
On the witness stand Monday, Gates spoke about how he broke the law with Manafort, including by providing false and doctored documents to banks in an effort to inflate Manafort's income to qualify him for loans.
Gates also testified that both he and Manafort had control over the foreign accounts, as well as Manafort's business associate Konstantin Kilimnik.
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.
But defense lawyers are trying to convince the jury that Manafort was consumed by his consulting business and left the particulars of his finances to professionals and, in particular, to Gates.
In court on Monday, Gates testified that he did steal from Manafort. But while Laporta acknowledged that she regularly communicated with Gates, she said she believed Manafort was in the know.
Prosecutors allege he dodged taxes on millions of dollars he made from his work for a Russia-friendly Ukrainian political party. Manafort, 69, meanwhile, is facing 18 felony charges that could carry a maximum of 30 years in prison.