Employees were able to see which videos affiliated users watched through a dashboard. The employees could also see the users' ID numbers.
Gershkovich was arrested in March and accused of espionage, which he and his newspaper, the Wall Street Journal, deny.
The Kremlin has said Gershkovich, the first US journalist detained in Russia on espionage charges since the end of the Cold War, was caught "red-handed."
The hearing is essentially procedural - about how Gershkovich should be detained as he awaits trial, not about the substance of the charges.
A Moscow court next week will hear an appeal by Gershkovich's legal team against a ruling that he be held in pre-trial detention at Moscow's Lefortovo prison until at least May 29.
Russia has presented no evidence to support the case against Gershkovich, which is proceeding in secret.
The Kremlin said Gershkovich had been carrying out espionage "under the cover" of journalism.
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