Prosecutors were expected to ask Gates for an inside look at what they say was a scheme by Manafort to use offshore bank accounts, shell companies and wire transfers as to avoid hundreds of thousands of dollars in tax bills.
Gates was Manafort's right-hand man during the pair's lobbying work in Ukraine. Gates said "yes" to both questions.
Gates already pleaded guilty in February to one count of conspiracy against the United States and another count of making false statements to federal investigators.
"Rick Gates had his hand in the cookie jar and couldn't let his boss find out", Manafort defense attorney Thomas Zehnle said during opening arguments.
Gates also reportedly told the court on Monday that he had been told by Mr Manafort not to disclose foreign bank accounts.
Mr Manafort's attorneys signalled they would seek to blame Mr Gates and had accused him of embezzling millions of dollars from Mr Manafort.More news: Facts of Life star Charlotte Rae dies at 92
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Juries are typically skeptical of witnesses who enter into agreements with prosecutors, said Sam Buell, a former federal prosecutor and a white-collar law expert at Duke Law School, so he said he was not surprised by the defense's strategy of going after Gates.
"This is typically the most important testimony that a "flipper" can give prosecutors - an admission that they committed crimes along with the defendant, along with an explanation of how they did it", Mariotti tweeted on Monday.
He also admitted to concealing millions of dollars in foreign bank accounts on Manafort's behalf and to falsifying loan applications and other documents to help Manafort obtain more in bank loans. The charges largely predate his five months on the Trump campaign but were the first to go to trial arising from Special Counsel Robert Mueller's investigation into Russian interference in the 2016 USA election. Similar to prior witnesses, Laporta testified that Gates and Manafort were in lockstep but that Manafort was in charge.
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 coordinated with Russian Federation - the central question Mueller's team has tried to answer.
On Friday, a tax preparer named Cindy Laporta admitted that she helped disguise $900,000 in foreign income as a loan in order to reduce Manafort's tax burden.
All told, prosecutors allege that Manafort failed to report a "significant percentage" of the more than $60 million they say he received from Ukrainian oligarchs. It's the first of Mueller's prosecutions to reach a jury, but lawyers have made no mention of President Donald Trump or possible campaign coordination with the Kremlin.