Many home-brewed Game Cube games are distributed as ISOs. An ISO is a disc image file. Unfortunately, not all emulators can support ISO files. To use the file, you must first convert it to GCM format. Since the two formats are identical in all but name, you can convert the ISO by renaming it.
Right-click the ISO file's icon and select "Rename" from the context menu.
Delete "iso" from the file name and replace it with "gcm."