Mr. Gates was a religious college student when he first came into Mr. Manafort’s orbit in the mid-1990s accepting an internship as a researcher at a pioneering consulting firm co-founded by Mr. Manafort that did political and lobbying work for corporations and foreign politicians, ranging from the lottery operator Gtech to the Trump Organization to the Philippine dictator Ferdinand E. Marcos.
Mr. Manafort left the firm the same year Mr. Gates arrived. But they continued traveling in the same circles as Mr. Gates impressed remaining partners, who he occasionally chauffeured between the firm’s Alexandria offices and meetings in Washington.
“Very smart. Good work ethic. He was a guy I thought would go places someday,” said Charlie Black, a co-founder of the firm, who offered Mr. Gates the internship at the recommendation of a friend, and then hired him full-time afterward. “I didn’t know where he would end up, but I always liked him. I still do.”
Mr. Gates joined up with Mr. Manafort in 2006 to help with business ventures in Ukraine that sprang from Mr. Manafort’s work on behalf of Russia-aligned oligarchs and the politicians they supported, including the man Mr. Manafort helped elect president, Viktor F. Yanukovych.
Mr. Manafort provided the connections and signed off on all the big decisions, but it was Mr. Gates who devised much of the strategy and administered the business, according to people who worked with the pair there. They said Mr. Gates did everything from approve expense reimbursements to draft business plans.
That symbiotic relationship carried over to Mr. Trump’s presidential campaign.
In the courtroom testimony on Monday, the prosecutors elicited testimony from Mr. Gates and from one of Mr. Manafort’s accountants that tied Mr. Manafort more closely to Russia. The accountant, Cynthia Laporta, testified that in 2006, Mr. Manafort received a $10 million loan from Oleg V. Deripaska, a Russian oligarch close to President Vladimir V. Putin of Russia. She said she saw no evidence it was ever repaid.
And Mr. Gates testified that Konstantin V. Kilimnik, a Russian citizen who prosecutors claim is tied to Russian intelligence, had signatory authority over some of Mr. Manafort’s hidden accounts in Cyprus.