Iranian news website PressTV reported that among those attending were Turkmenistan's President Gurbanguly Berdimuhamedow, Equitorial Guinea President Teodoro Obiang Nguema Mbasogo and Russian President Vladimir Putin.
At the conclusion of the summit, it is hoped that member states will be signatories to the Tehran Declaration.
As the event's name suggests, it is the third meeting of its kind with the inaugural summit hosted by Qatar in 2011, and the second in Moscow in 2013.
The GECF aims to promote the right of its members to independently plan and manage their natural gas resources, as well as being a forum for the exchange of ideas and information, and coordination of gas-related developments.