On importing gbXML files from Revit 2013 into Ecotecty Analysis you are faced with the following error message:

This error is being presented because the gbXML export file is not encoded in a format Ecotect recognises (and for some reason triggers an IFC error message).
Unlike earlier versions, Revit 2013 exports in UTF-16 format whereas Ecotect expects UTF-8.
The simplest way to work around this is to open the gbXML file in Windows Notepad and save the file, ensuring you change the encoding format to UTF-8.

The gbXML should then import without problem.

Via BIM Toolbox ..