It's also really lucky that the Gamecube GPU, released well over 10 years ago, matches extremely well to the modern GPU pipeline.
Compare this with the PS2, which has a bunch of crazy programmable coprocessors to build an extremely flexible but hard-to-emulate system.
It's probably no coincidence, given that it was developed by ex-SGI engineers who founded their own company, ArtX, who got bought out by ATI to develop the R300, one of the first modern programmable GPUs.