The case pits two of Trump's former senior campaign aides, men who worked together for decades, against each other in a high-stakes battle that could land either or both of them in prison for years.
Gates, 46, is already facing up to six years in prison under the terms of a plea deal he struck with prosecutors in February when he pleaded guilty to two charges of conspiracy and lying to the Federal Bureau of Investigation.
Later, when Yanukovych fell out of power and fled to Russian Federation, the business struggled and prosecutors say Manafort lied to banks to obtain loans to help maintain his life of luxury.
Gates has read off the names of more than a dozen shell companies he and Manafort set up in Cyprus, St. Vincent and the Grenadines and the United Kingdom to stash the proceeds of Manafort's Ukrainian political consulting work. They were the first two people indicted by Mueller's grand jury.
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.
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 revealed that Pinchuk paid Manafort through a company called Plymouth Consultants Ltd. for what he described as a legal project.
Prosecutors have accused Manafort and Gates of conspiring to hide a significant portion of the $60 million that Manafort earned through his political lobbying in the Ukraine for its then-President Viktor Yanukovych. When Robert Mueller began investigating Russian interference a year ago, Manafort's questionable lobbying work - the sort of activity that has historically rarely been investigated in Washington - was put under a microscope, exposing him and his associates.
In his testimony, Gates said, "I was the one who helped organize the paperwork".
During the testimony, Manafort did not stare Gates down as he did on Monday.
Taking the stand on the trial's fifth day, Mr Gates admitted to helping Manafort doctor financial statements, hide foreign income and cheat on his taxes.
He testified that Mr Manafort had directed him to lower taxable income by reporting overseas income as loans. He also described in details how he falsified loan applications and other documents to help Manafort obtain more in bank loans. But Trump has shown interest in the proceedings, tweeting support for Manafort and suggesting that he had been treated worse than gangster Al Capone.
Gates is considered the "star witness" of the trial, which is in its second week.
One accounting trick saved Mr Manafort $US500,000 ($676,907) in taxes, she said.
Laporta thought that was odd, so she asked Gates for information about the $2.4 million Manafort was supposedly owed.
The testimony from Gates, a former adviser to Donald Trump, comes after reaching a plea deal with special counsel Robert Mueller earlier this year to testify against his former partner in a lucrative worldwide political consulting firm.