Gates is also admitting to carrying out a vast scheme to hide millions of dollars in foreign bank accounts on Manafort's behalf. Gates said "yes" to both questions.
Gates was expected to be a star witness in the government's case against Manafort having pleaded guilty in February and agreed to co-operate with prosecutors under a deal that could lead to a reduced sentence.
Manafort, facing charges of bank and tax fraud related to his work in Ukraine, has pleaded not guilty to the charges.
Gates told the jury that as part of his plea with prosecutors, he revealed several other crimes that were not previously known and that were not included in the federal indictments against him and Manafort.
Downing will probably focus on Laporta's interactions with Gates, who pleaded guilty and is cooperating with Mueller.
"They probably have hundreds and hundreds of emails", the former US prosecutor said.
Attorney Thomas Zehnle accused Gates of embezzling money from Manafort's consulting firm and said that Gates was now agreeing to cooperate with prosecutors to cover his tracks.
The prosecution didn't confirm that Gates, a prosecution witness, would be next.
The emails will provide the jurors a glimpse into the private conversations between Manafort and Gates, the man to whom he entrusted his personal and business finances.
Earlier Monday, Ellis threatened to kick reporters out of the courtroom if they continue being "disruptive".
"Were you involved in any criminal activity with Mr. Manafort?" a prosecutor on special counsel Robert Mueller's team asked Gates, the key witness in the charges brought against Manafort. 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.
Attorneys representing Manafort have built their defense argument around Gates, painting him as the true culprit behind their client's alleged crimes and asserting that Manafort's only mistake was "placing his trust in the wrong person", referring to Gates.
Under cross examination, she said at the time she believed Manafort was directing Gates' actions and "knew what was going on".