With the growing concerns for environmental deterioration and sustainable development, multiple energies, e.g., electricity, gas, and heat, are coordinated for more efficient energy utilization. Being scheduled under the multienergy demand response framework, the demand side resources (DSR) can be used to assist the power system operation by providing equivalent operating reserves (ER). However, in previous studies, the characteristics of the DSR are not fully transparent to the system operator, which will complicate the central dispatch of the power system. This paper proposes an original modeling technique that uses virtual gas-fired power plant (VGPP) to model the DSR flexibility in terms of providing ER. The characteristics of VGPP, including the dispatchable capacity, gas consumption curve, and cost curve are thoroughly studied. Moreover, the proposed modeling technique is incorporated in the optimal operation of the power system, and its effectiveness is validated in the case study.