Gamepedia now has a dedicated imported for Game Collector.
Export your data as XML from Game Collector.
Since Game Collector is a Windows program, you will have to use find and replace on the XML file first to update the path to the covers folder in order to import the covers. In the XML the path points to the Windows location:
<coverfront>C:\Program Files\Collectorz.com\Game Collector\Data\Images\hackQUARANTINEPart420614_f.jpg</coverfront>
You would update that to point to a copy of the Images folder on your Mac:
Placing the "Images" folder on the desktop the find and replace to execute would be as follows (where username is your user on the Mac):
C:\Program Files\Collectorz.com\Game Collector\Data\Images\