Yeah, and the GPU code is designed to read the linux kernel into RAM, and set it up so it begins executing the linux kernel as the inital instructions of the AR

Yeah, and the GPU code is designed to read the linux kernel into RAM, and set it up so it begins executing the linux kernel as the inital instructions of the ARM CPU. If you want some more normal bootloader like u-boot you need to jump through hoops to make sure the GPU based bootloader can treat it like a weird Linux Kernel. (In theory, with source for the GPU 2nd stage bootloader one could change things, but RPI foundation does not provide access to that source).