Hi James,
I think .gem files are geometry files, maybe something to do with
IES <ve>? (Their site seems to be down at the moment!)
I believe gbXML is the 'standard' file format for transferring an ArchiCAD model into an energy analysis program. Ralph Wessel (Encina software) on these forums is developing a gbXML exporter at a reasonable cost. Maybe send him a PM?
It might be best to ask your energy consultants exactly what software they are using to save any confusion. I suppose alternative formats to check with them may be 3d .dwg or .IFC also, but I should imagine gbXML is more likely.
Good luck!
Edit - Try this link to IES instead -
http://www.iesve.com/UK-ROI
Stupidly designed front page doesn't work on previous link!