I was wondering if I could make a game that requires the Game Link Cable™.  I read the rules a there are mentions that forbids game that requires add-ons (hardware that enhances the console) but there are no mentions about accessories (non-essential hardware that adds new features).

I am not sure, but think it is possible, but you are limiting your audience. maybe the judges wont be able to properly evaluate multiplayer games...
but the same could be said for color-only games...

You may include link cable support in your entry, but it should not be required in order for the judges to play it. This would be considered "extra hardware and an add-on device". You might allow this setup to unlock additional features, but should not rely on the judges having two Game Boys and a Link Cable.

• Hardware #13: Entries cannot rely on extra hardware or add-on devices, they must be playable without a specific setup.