From a quick look at the Small Office DOE prototype, seems it used an AirLoopHVAC:UnitaryHeatPump:AirToAir with a Single Speed DX Cooling Coil, a Single Speed DX Heating Coil, and a gas supplemental heating coil.

In the OS Library, you won't find this exactly, the closest you have is one where the supplemental heating is electric, called "Unitary - DX Heat Pump - Cycling - Elec Reheat".

Couple of things to note:

• I would advise against using an Unitary System for this unless you specifically need to, you'd be shooting yourself in the foot in my own humble opinion: it's much more complicated to work with. You can model a PSZ using a regular air loop. You can click to create a new system, and select "Packaged Rooftop Heat Pump" in OS App, then just change the supplemental electric heating coil with a Coil Gas instead:

• If you really want to create one with a gas coil for supplemental heat, you'll need to write a script or measure for this, or better yet, you could also just use the Create DOE Prototype Building measure on BCL (developped as part of openstudio-standards) to generate the SmallOffice Prototype in an OpenStudio Model directly. Here's one of the PSZ loops for the SmallOffice prototype generated using the BCL measure.

You can then clone that into your specific model using the command line or a measure. Note that there no point trying to clone it into your own hvac_library.osm currently as it would be there but you won't be able to select it in the OS App, that would require adding a new line to the source code (here)