This study addresses the importance of initial GOR in SAGD heavy oil recovery operations. By history matching two laboratory scale experiments, one with dead oil and the other with live oil, we corroborated the theoretical and numerical prediction that gas would accumulate at the front of a steam chamber. This gas accumulation could slow down oil production as well as heat loss to the overburden. It is suggested that monitoring gas production during SAGD field operations may be critical for investigation of impacts of gas, and for developing strategies for performance improvement.