You can follow this thread that is currently under development. GPT/BIOS boot is possible, but I would leave that hornets nest afap and set up for EFI only if I were you.
Not entirely sure about that. I have not used GPT/BIOS mode but following the discussions on the forum about this right now seems to indicate not specifiing partition but rather just the device, ie sda instead of sda1 could be the way to go.
But as I said, I would leave that weird setup afap and go for efi only.