With the provided "uninstall.exe" file you can safely remove the map from Basecamp/Mapsource. This file is located in the same folder as the map tiles (c:\garmin\openstreetmap\openfietsmap). In the latest versions you can also remove it by opening the Windows Control Panel and using the Add/Remove Programms.
With Mapset Toolkit from http://sites.google.com/site/cypherman1/ or Javawa GMTK you can repair problems with the installation of maps:
With "check registry" (right panel below) you can see which installed maps cause problems
and with "Uninstall" the application can be removed.
Uninstall gmap version
To uninstall it, simply remove the gmap folder (or rename the gmap extension)