You can find a comprehensive listing of group codes in the AutoCAD help system. Here are instructions for finding the group codes.
1. In AutoCAD, choose Help > AutoCAD Help from the menubar
2. Click on the Contents tab of the Help Topics dialog box, then double click on Visual LISP
3. Double click on Shortcut to AutoCAD Visual LISP Help
4. Double click on DXF Reference, then again on Chapter 7: OBJECTS section.
5. Double click on Common Group Codes for Objects.
You will see a listing of group codes and their meaning.