this is the procedure
1. Run the GMapTool.exe
2. Hit "Add Files" and add the "gmapprom.img" file
2.5. Under the "options" tab tell it the location of cgpsmapper.exe
3. Click on "Split" at the top.
4. Select the directory where you want to save it
5. Make sure that "Create" is set to "files for mapsource"
6. Make sure there is a tick in "compile map preview"
7. Mapset name is "xxxxxxxxxx"
8. Tick "set mapset FID"
9. FID is xxx and PID is x
10. Click on "split all".
11. Wait until it is done, should take a couple of minutes.
12. Close GmapTool.exe
13. Open the directory from step 4.
14. Locate install.bat and run it.
15. That should be all you need to do to install xxxxxxx map into mapsource.

thanks to original poster of this guide