This is a problem with many cad programs. FreeCad assembly model is made from individual parts filed in step. FreeCAD file format is not read by gabCAD so I can not put an assembly into the gallery viewer (not listed). Is there a utility (free open source) to create an assembly step file from multiple part step files?
a) now (05/2017) GrabCAD does support FreeCAD files
b) STEP AP203 does not support assemblies, STEP AP 214 does
(you can switch between the two using "Edit > Preferences > Import/Export, while making sure, draft workbench is selected)
c) haven't tried that yet, but OBJ should support assemblies