Dear Amr,
As of SAM 2017.1.17, SAM's battery model is available with the Detailed Photovoltaic model when combined with the PPA Single Owner financial model, which means that you can use it to model a power project with no building load, i.e., one that is not behind the meter.
In that configuration, only the manual dispatch mode is available, so you have to manually define the charge and discharge schedule of the battery using the six dispatch periods and weekday/weekend matrices on the Battery Storage input page. The system's electrical power output is the sum of the PV power and battery power. (The battery power is negative when the battery is charging.)
You could model the PV/battery system in this way, and then outside of SAM, determine how the gas plant would need operate given the PV/battery system's output.
You might also consider using HOMER for this kind of analysis. It is designed to model power systems with a mix of different resources, and is useful for analyzing different dispatch strategies.
Best regards,
Paul.