The Game cube has a video output wire that looks like this : How can read the video input on an iMac with this? What do I need to buy? What software should I use?
If you have an iMac with a Mini-DisplayPort or Thunderbolt port (21/27 inches) .
You can use your iMac in target screen mode using this kind of adapter : Mini-DisplayPort to Composite
If your iMac is older (20/24 inches) you won't be able to use your iMac with your GameCube.