Here is a link to our current gbXML import/export. @macumber is on vacation but when he gets back I can follow up with him about the plans for gbXML schema certification.

gbXML import (geometry, constructions, thermal zones, and schedules only)
gbXML export (geometry, constructions, and thermal zones only)

Constructions should be supported, but there may be an issue related to that. If you have a sample gbXML file files with constructions please send it to OpenStudio@NREL.gov and we can use it internally for testing.internally.