I don't know of any program that will convert a .DWG file directly to Gerber. You would have to use one of the many software programs that convert .DWG to .DXF first. You would then have to read the .DXF file into a CAM program such as Gerbtool or CAM350, and output the Gerber file.
If you want, you can PM the file to me. I can try to generate a Gerber for you.