Does gbXML support HVAC systems?

Yes, gbXML does support HVAC system elements that are now better mapped to the HVACTemplate objects in EnergyPlus. If you display the schema documentation (download here), you will notice that there are 2 main elements called AirLoopEquipment and HydronicLoopEquipment. These two elements include a number of child elements that allow you to model entire air and hydronic HVAC systems.