USB Gecko communicates with a GameCube or Wii via EXI bus. In essence, EXI bus acts like well known SPI.
If command names are confusing to you, read them as if you were issuing commands from PC to console i.e. 0xA Receive Byte means "read 1 byte from GC".
These commands were documented by studying Shuriken USB VHDL code. The official USB Gecko and USB Gecko SE supported more commands like 16 bit reads/writes or NOR flash access.