Playing with gcm files

I've been playing with gcm files lately (GameCube games) and learning a lot about them. There seems to be little information about them and a lot of disinformation so I thought I'd share what I've learned so far.

Basically the GameCube .gcm format is similar to an iso, but it's really a proprietary format. Each disk is 1.4G and most games don't use the whole disk but there's seemingly random data floating between the files. This garbage data doesn't compress well, so people have developed wipers which change the garbage data into zeros which compress very well. The biggest example I've seen is Animal Crossing, which shrinks down to a whopping 20M when wiped. The gcm file is still 1.4G large though, so another option is to bunch the files together (kind of like a defrag) then chop off the garbage. This is a little more risky since it alters the Table of Contents, but if done correctly should have no negative effects.

Since wiping games makes such a big difference in download size I've started wiping each game in the GameCube Vault. I'm also making sure each game is 1.4G in case that matters, since I've heard it must be 1.4G to burn to disk (though I haven't confirmed that). I've found some games break when I wipe them though, so perhaps they weren't copied properly? Whatever the case, wiping the garbage will save me a bunch of disk space and make GameCube games download faster with no side-effects, so it sounds like a win-win.