The filesystem is also completely proprietary. http://hitmen.c02.at/files/yag... has a description of GameCube discs; http://wiibrew.org/wiki/Wii_Di... has Wii discs. (Wii discs are similar to GameCube, but it supports multiple partitions and offsets are multiples of 4 in order to address a full 8 GB dual-layer DVD using 32-bit values.)
The reason why most people think GameCube and Wii discs are written "backwards" is because the disc mastering tools deliberately pad the beginning of the disc with filler data in order to push the actual data towards the outer edge of the disc. This is because the CAV drives used in GameCube and Wii are able to read data faster if they're closer to the outer edge.