Gates' testimony was part of the prosecution's effort to prove that Manafort was responsible for financial manouverings that he and other witnesses have testified include filing false tax returns and failing to report foreign bank accounts.
Manafort's Kiev-based aide Konstantin Kilimnik was able to collect $500,000, Gates said, but "to my knowledge it was never paid in full".
Throughout the trial, they have tried to pin the blame for financial misdeeds on Gates, whom they also have accused of embezzling millions from Manafort's consulting firm.
Mr Gates, who first met Mr Manafort working for him as an intern fresh out of college, has been described by witnesses as his right-hand man in his multimillion-dollar political consulting business.
The scene was less tense Tuesday when Gates resumed his testimony and Manafort's lawyers prepared to grill him on cross examination.
Those admissions are likely to become a focal point for Manafort's defence team when he is cross examined on Tuesday.
The testimony came after Gates pleaded guilty in February and signed a plea deal to testify against Manafort, his former partner at an worldwide political consulting firm. 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.
Special counsel Robert Mueller is investigating whether any Trump aides colluded with an alleged Moscow plot to help elect him as USA president, and whether there was any obstruction of justice.
The charges relate to Manafort's work for Ukraine's former pro-Kremlin president, Viktor Yanukovich. Mueller is also investigating possible coordination between Trump campaign members and Russian officials in the election campaign, but the charges against Manafort do not address that.
Un médecin s'attaque à Jérôme Cahuzac pour l'empêcher d'exercer
Il en appelle au Conseil de l'ordre des médecins, estimant que le casier judiciaire de l'ex-ministre devrait lui interdire toute pratique, comme le raconte le " JDD ".
Gates and Manafort worked closely together for years. "In reality, it was basically money moving between accounts", Gates said, according to the Associated Press. In exchange for his plea, Gates agreed to testify against Manafort.
This courtroom sketch depicts Rick Gates, right, answering questions by prosecutor Greg Andres as he testifies in the trial of Paul Manafort, seated second from left, at the Alexandria Federal Courthouse in Alexandria, Va., Monday, Aug. 6, 2018.
Laporta detailed multiple examples in which Manafort and Gates sought to doctor financial records - first to lower Manafort's taxable income and then later to inflate his income so that he could get bank loans, she said.
Gates, frequently pictured with a thick lumberjack beard at the height of his political power alongside Manafort, entered the courtroom at 4:15 p.m. Monday, clean-shaven and wearing a dark suit and gold tie.
When the trial broke for lunch Tuesday, Manafort looked back at his wife, sitting in the front row, smiled and winked at her, followed by a quick shake of his head, seeming to indicate he was unfazed or unbothered by the morning's testimony.
On Monday, U.S. District Judge T.S. Ellis repeatedly clashed with prosecutors about the relevance of such testimony and once again urged them to speed things along. Similar to prior witnesses, Laporta testified that Gates and Manafort were in lockstep but that Manafort was in charge.
But she said she was under the impression that Mr Manafort had been directing his deputies' actions and "knew what was going on".
The government called Gates, its star witness, to the stand Monday, just five days into what prosecutors originally expected would be a three-week trial. Laporta testified that she saw no evidence that the loan was ever repaid.
But Mr Manafort is not charged with helping the Kremlin.