... as long as the card has a working UEFI GOP
Reflecting on this...
OpenCore and rEFIt variants such as RefindPlus are not tied to GOP and will work just as well when GOP is absent but UGA is present. That is, they simply need a working graphics protocol, which can be GOP or UGA. On the other hand, the default Apple Boot Manager, StartupManager, apparently only looks for UGA (on cMP at least).
The graphics protocol on the GPU, GOP or UGA, is what is used throughout once the Mac is started and there are not multiple instances of GOP used for different phases with the one supplied by the GPU, often referred to as UEFI-GOP, being a sort of basic/limited one that is meant to only be used in preboot for instance.
Well, the GPU will indeed be in a limited init state meant only for basic/preboot graphics but what happens is that the GPU is partially initialised as part of the Mac power up process (enough for hardware tests etc) and the graphics protocol from the GPU, GOP or UGA, is available around when the chime sounds following this first partial init. Later, the OS kernel connects/reconnects the graphics protocol and completes GPU initialisation by loading drivers/kexts that activate baseline GPU acceleration (for a lack of a better description).
That is, the reason for a difference in graphics performance when running such a GPU on something like El Capitan or Yosemite as compared to Big Sur or Mojave is down to whether the "baseline GPU acceleration" is present (due to the required drivers/kexts having been loaded) or absent.
As to why we get a blank screen until Mac OS loads with some GPUs such as the RX 580? Well, since StartupManager only looks for UGA and those GPUs do not have UGA, there is no graphics output until the GPU's graphics protocol (GOP for such GPUs) is connected/reconnected by the OS later. OpenCore etc simply just use the GOP from the GPU in such cases (might need to make it actually available on some Macs first).
I suppose we could understand "UEFI-GOP" as the state of the GPU before full init and I cannot think of another short and sweet term to use but slight disquiet I have is that it is often stated that tools such as OpenCore and the rEFIt variants can activate a "UEFI-GOP" which can then be used by the OS later and this is seems to imply the multiple GOP thing.
What those utilities do is to connect to the graphics protocol provided by the GPU and whatever the GPU has provided will ultimately be used by Mac OS later and the GPU accelerated, at least at the "baseline" level, by the mac OS if the drivers/kexts are available. If not, the GPU will work without any acceleration at all (same as it would for the pre-boot screen).
In some cases where GOP is present on the GPU but fails to be loaded by the GPU for misc reasons, such as was an issue with the
MacPro5,1 (has a "Mutant" ConsoleOut GOP that needs handling) as well as on some Nvidia GPUs on all cMP, they can bypass the issue, trigger a reload of the protocol, and if successful, proceed as normal with the OS also proceeding as normal later.
Having rambled on this, it might still just be easier to say "UEFI-GOP"!
EDITS/NOTES
A: Maybe "EFI Graphics" or "Basic Graphics"?
B: "OC activates UEFI-GOP" likely from need to fix MP51 ConOut. Not needed on MP31 but cMP = MP51 for many
