For the Gamecube, I'm not quite sure. I have never modded any of my systems. I was always hesitant, as I didn't want my system messed up. Especially my Nintendo systems. I have gotten those Pro Action Replay's before, and those worked well. They were just carts that you plugged into the back port of the system. No need to open up the system. You can plug it in from the outside. So those are the ones that I always felt comfortable with when I wanted to play the Japanese import games. I did it a lot for the PS and PS2 games. My friend did it for those, and for his Sega Saturn. Haven't seen a Pro Action Replay used for game systems since those days, tho I'm sure they still exist.
As for modding the systems, my old roommate had a friend who did all of that. He used to mod everybody's system. I've had chances to get mine modded, but I, again, was always hesitant. I didn't want to ruin my system. He promised that if he messed up my system in any way, that he would buy me a brand new one himself. I was tempted, but never bit the bullet on it. But some of my friends did. Mostly with the Xbox and Playstation consoles. But, that roommate moved out, got married, has some kids now, and moved to the far end of town, and is very hard to get a hold of, as he hates to answer his phone, and its rare that he does. But, honestly, I'm not sure if he's still in contact with that modder or not. It's been like 10 years or so, now.
Of course, you can probably still find "Free Loaders". I'm sure that they made a Gamecube version. Basically, just like Pro Action Replays, you'd load up a disc, and then switch out discs to the import version. One way was to load up the Loader disc, to trick the system into reading the region code on that disc, and then there's a 'pause' menu type screen where you can pop that disc out and put in your import disc, and it'll continue the game, without reading the region code on the import disc, as it's already read it on regional disk. Then you can the import game normally.
There is no modding of the system of any kind. None. Just swapping disks. That's all. That sounds like all that you need. So you can look up "Free Loader" disks. Then you won't have to open up your system or mod it in any way. Those are the ways that I have done it in the past. The PS2 was the last system that I did that for. But the principle is the same. Again, I always refused to get my systems modded, as I was always too worried. Even when I watched this guy mod my friend's systems, and then got to play on their systems, and see it all first hand work perfectly. I just couldn't bring myself to ever modding my system. So the Pro Action Replays and Free Loaders (HD Loader for the PS2 - Hard Drive Loader, that is).
The HD Loader for the PS2 was neat. You just plug a harddrive into the back of your PS2. The older fat version. And then, pop in HD Loader, and it installs onto the harddrive. Then, with that program running, you pop out that disk, and then pop in your disk, and it'll burn your disk onto the harddrive. You just pop in all of your games that you own, one by one, and it'll burn them all onto the harddrive. Then, all you need to do is just keep the HD Loader disk into your system, and you can play all of your games off of the harddrive itself, instead of switching disks. That also lowered the loading times on games too. So that was neat. But yeah, it's like having one "master disc" for all of the games that you owned. Didn't work with the newer, slimmer model, as the slimmer model didn't have a port for a Harddrive. But it was a neat feature for the older model.
But yeah, Free Loader or Pro Action Replay, or something like that. Check out those, and you won't have to mod your system in any way in order to play import games. I used to import a lot of games. Mostly for the PS and PS2. But yeah, I refused to open up my systems for any reason. So look that up, and try that out. Good luck.
