Yes, I have that watermark on my Windows 8.1 screen, and I had Windows 8 installed in UEFI booting mode. That remained after the 8.1 update.
I have not enabled Secure Boot, since that is not necessary to enable the Windows 8 Ultra Fast boot mode. I don't use a Gigabyte board, but their UEFI/BIOS may have the same option, CSM. Disabling CSM enables UEFI booting, and is independent of using Secure Boot. OTOH, enabling Secure Boot will automatically disable CSM. That is how some guides tell you to enable UEFI booting, and it works. But you do not need to use Secure Boot to have a UEFI booting PC. That is, as long as your board's UEFI/BIOS has the CSM option available.
The issue with not being able to get into the UEFI/BIOS with Secure Boot is really just the side affect of Ultra Fast Boot (UFB) being enabled by enabling Secure Boot. UFB either bypasses the ability to enter the UEFI/BIOS by pressing Del, etc, or it's just to fast for us to press the key in time. Again, Ultra Fast Boot is independent of Secure Boot, if your board's UEFI has the right options available. Windows 8 has an option to restart a PC directly into the UEFI/BIOS when UFB is active.
My ASRock boards have Secure Boot, CSM, and Fast Boot (mode) as separate options. So I can toggle UEFI or BIOS/legacy booting with the CSM option. I can choose UFB, Fast Boot, or Disabled with Fast Boot mode. Secure Boot and installing its keys is available.
As you said, in order to UEFI boot, enabled via Secure Boot (or CSM alone) the video source must support the UEFI GOP protocol. Otherwise after POST, no video, a black screen. My ASRock board catches this problem, and displays an error message, so at least you know what happened.
The Intel on-CPU graphics supports GOP, as far back as Sandy Bridge CPUs/HD Graphics 3000 at least.
I don't know to what extent AMD integrated graphics support GOP, or AMD/ATI video cards.
Nvidia GTX 700 and 600 series cards can support GOP, with the correct VBIOS, as you said. All earlier Nvidia cards, the 500 series and earlier, cannot be updated with a VBIOS to support GOP. It depends on the manufacture of your Nvidia based 600 or 700 card if the GOP VBIOS is standard with the card. I know all EVGA 700 series cards support GOP out of the box. EVGA 600 series cards all need a VBIOS update to support GOP, which EVGA will send you on request.