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 Topics from the menubar
2. Click on the Contents tab of the Help Topics dialog box, then double click on Customization Guide
3. Double click on Appendix C- DXF Group Codes
4. Double click on ENTITIES Section.
You will see a listing of group code topics shown below. Click on the button next to the topic for which you need information. Sub-heading under Common Group Codes for Graphical Objects and Dimensions are also shown here for your convenience.
ENTITIES
Common Group Codes for Graphical Objects
Common Group Codes
Acad Reactors
Acad xdictionary
3DFACE
3DSOLID
ARC
ATTDEF
ATTRIB
BODY
CIRCLE
DIMENSIONS
COMMON DIMENSION
Aligned, linear, and rotated dimension
Linear and Rotated Dimension
Radial and Diameter Dimension
Angular Dimension
Ordinate Dimension
Dimension Style Overrides
ELLIPSE
HATCH
IMAGE
INSERT
LEADER
LINE
LWPOLYLINE
MLINE
MTEXT
OLEFRAME
OLE2FRAME
POINT
POLYLINE
RAY
REGION
SEQEND
SHAPE
SOLID
SPLINE
TEXT
TOLERANCE
TRACE
VERTEX
VIEWPORT
XLINE
ACAD_PROXY_ENTITY