Playing import games on a Gamecube is very simple. You’ll just need to buy a Freeloader disc which is available from places like Ebay and Videogameimports.com. Put your Freeloader disc in first and when prompted on-screen, take out your Freeloader disc and put your import disc in.
The success rate of this disc is very high – the only problems I’ve heard about so far are with ‘Mr Driller:Drill Land’ and ‘F-Zero GX’, – the latter of which is available on PAL now anyway.
It’s recommended that you keep a separate memory card handy for saved games for Japanese games as there’s a chance that a Japanese game will format a card with US/UK saved games thus erasing all your hard work.