You cannot export devices from a certain GC as a way of filtering. In the .modeliggawayresource.xml there's a section where you have to specify which container you want to be exported. All your devices would have to be in that container. By default, Universe is selected.
<!-- RootContainerToExport specifies the root container in -->
<!-- SPECTRUM to be exported. -->
<!-- -->
<!-- ******************************************************** -->
<RootContainerToExport model_type="Universe" model_name=""/>
In the ExportConfiguration section you can specify what is that you want to export. In the DeviceExportAttributes section you can set the attributes to be exported.
Just go over the config file, you'll figure it out. It's self explanatory.