Gates, who also served in a senior role in Donald Trump's presidential campaign, is expected to face aggressive cross-examination once prosecutors are finished questioning him.
Under cross examination, she said at the time she believed Manafort was directing Gates' actions and "knew what was going on".
Manafort also told him to wire some of his political consulting income to his bank accounts in the United States, Gates added.
"Did you commit any crimes with Mr. Manafort?"
Manafort, 69, has pleaded not guilty to 18 counts of bank fraud, tax fraud and failing to disclose foreign bank accounts. President Trump has previously spoken of the Mueller investigation, referring to it as a "witch-hunt", and reiterating that Manafort only worked for his campaign for a few months.
His testimony represents the biggest test yet for Mueller's investigation even though this case does not directly play into the issue of alleged cooperation by the Trump campaign in Russian election interference.
The jury has heard how Manafort made tens of millions of dollars for work with pro-Russian politicians in Ukraine.
As well as serving as Manafort's right-hand man in their multimillion-dollar business, Gates was also Manafort's deputy on the Trump campaign.
Gates and Manafort were charged with a combined 32 counts of tax and bank fraud in February, after which Gates accepted a plea deal which included testifying against Manafort.
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Gates admitted in testimony on Friday that he had lied to Laporta, as well as other accountants and Manafort's bookkeeper.
Ms Laporta detailed multiple examples in which Mr Manafort and Mr Gates sought to doctor financial records, first in order to lower Mr Manafort's taxable income and then later to inflate his income so he could get bank loans.
U.S. District Judge T.S. Ellis III, who repeatedly interrupted prosecutors last week as they tried to present evidence about Manafort's lavish life such as $900,000 in expensive suits and a $15,000 ostrich jacket, has clashed with prosecutors, urging them to hurry up presenting their case.
As a sober Gates recited his testimony, Manafort reacted about the way you'd expect.
Accountant Cindy LaPorta - who was granted immunity from prosecution - was asked by prosecutors if she had been aware of a $10m (£7.7m) loan in 2006 from Russian oligarch Oleg Deripaska.
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Prosecutor Greg Andres pointed out that he had created a "loan forgiveness letter between Mr. Manafort and Mr. Manafort".
Laporta thought that was odd, so she asked Gates for information about the $2.4 million Manafort was supposedly owed.
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.
On redirect, prosecutor Uzo Asonye used a spreadsheet to show that Manafort's firm had earned $31 million in gross receipts between 2010 and 2014 - half the amount prosecutors say he netted during the same time frame in Ukraine.
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