Since EnergyPlus doesn't have a direct model for this, I think you would want to use the regular VRF system system, and set the type of fuel then do some postprocessing to be natural gas in the change outdoor unit. unit's electricity use to gas. See the Input/Output reference for ZoneHVAC:TerminalUnit:VariableRefrigerantFlow and AirConditioner:VariableRefrigerantFlow. There is an input field called "Fuel Type" in the AirConditioner:VariableRefrigerantFlow object that can be used to model a gas powered VRF system. AirConditioner:VariableRefrigerantFlow