A gcd filename extension belongs to Generic CAD which is a program completely separate to AutoCAD. I think your best bet is to try and get access to Generic CAD and save your drawing as a DXF. It looks like you can get a demo of its successor at http://www.generalcadd.com/downloads.htm which might be worth a try. AutoCAD will open a DXF with no problems, but it will open precious little else. LT is even more limited and will only really open DWGs and DXFs.