Come accross this problem? Here is the solution from the BIM Toolbox:

On import you are faced with the following error message:

Ecotect-gbXML Error-1

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.

Ecotect-gbXML Error-2
The gbXML should then import without problem.