Use BIOS GPU with UEFI Mobo+OS


I recently built a PC for GPU passthrough. As my host OS I plan on using Ubuntu 18.04 LTS which has been installed in UEFI mode. My motherboard is a Gigabyte X399 Designare EX set to UEFI boot.
My Vega 56 boots just fine but the moment I add my Gainward GTX 660 with BIOS firmware, my PC refuses to even show a POST-screen.

I am asking myself whether my GPU has to have UEFI firmware as well?
And if I activate Legacy boot mode does that have the same effects as booting BIOS directly? Meaning less than 2TB of storage per disk, MBR, none of the UEFI features such as SecureBoot and FastBoot?

I just wanted to check this before going out and buying a cheap UEFI-capable GPU.

Thanks for helping