I checked. Both wiitdb and wiboxart databases contain GameCube covers. Also titles.txt contains the ID list of GameCube games (the IDs starting with G).
So Wii Cover Hunter is already able to download Game Cube covers. Click on "path .wbfs" to enter in the title ID list, look for the titles of...
I have no knowledge about gamecube covers: is there a site like wiitdb where retrieve the titles list and the covers?
Are the gamecube titles indexed with a code, like wii ones?