On power-up the SoC's firststage bootloader reads the SD card to find bootcode.bin, and runs that on the GPU
This loads loader.bin, and runs that on the GPU
Then start.elf is loaded as the firmware for the GPU
None of these are open source - all are behind Broardcom's NDA
start.elf then loads kernel.img and starts Arm, and only at this point is there any possibility of open source. You can of course replace kernel.img with anything you like.