Well, someone once asked Nintendo about ripping, and based on the answer they got (according to my understanding) it was perfectly legal to rip a copy of your own game. Do some searching, and you can figure out how to do it. As for legally downloading GCM files, I don't know, but I'm pretty sure Nintendo would frown on it, even if you do own the actual game disc itself... I must admit though that I have a few GCM images, and no copy of the original game disc. I have them for different reasons than emulation, or even playing them, though (meaning, I don't have a modded GC either). I have the GCMs for the audio data they contain, not for the games themselves. Like Chibi Robo (yah, audio isn't that great either), Fire Emblem (I wish I had the actual disc, I'd like to play it), Sonic Adventures DX, and a few others.
So, for legality issues, I don't know. I do know that I get them anyway... Does that in any way answer your question in the way you wanted to hear? Mouser X over and out.