Select a floor plan view within Revit and then repeat your process...in other words, choose any floor plan, go to the tools|external tools|GBS and it will work. This is an intermittent item that periodically pops up.
This has to do with what's selected in Revit. Certain commands cannot be activated based upon the selected Element in Revit (Project Browser or Schedule View, for example). Select inside a Plan, 3D, Section, or Elevation View and you should be good.
Sustainable Design Product Manager
Kyle Bernhardt Product Line Manager Building Design Suite Autodesk, Inc.