The GrayFish code that's been analysed will only attack MBR-based disks. GPT disks on BIOS systems can have arbitrary code injected into them as well, but Windows won't run on GPT disks on BIOS systems so it's not really worth targeting. UEFI systems don't read a boot sector off a GPT disk - they run a bootloader directly instead. Without Secure Boot it's straightforward to modify that bootloader. It *shouldn't* be possible to execute arbitrary code from the UEFI variable store, but obviously there's always the potential for parsing errors or buffer overflows that could do something nasty.