ČEZ is seeking compensation for damages worth around CZK 1 billion (USD 45.4 million).
The outcome will be decided by a three-member legal tribunal.
The Kremlin-controlled Gazprom has the world's largest natural gas reserves and a monopoly on exporting gas from Russia through a network of pipelines.
Gazprom started cutting supplies to ČEZ, as well as to other companies in Europe, after EU countries imposed sanctions on Russia last year over the war in Ukraine, the report noted.