vBIOS = Video BIOS. Basically how GPU boots. Since used GPU are sourced from PCs, need to be flashed to match how Mac boot. It is a ROM image and need to be flashed into GPU cards onboard EEPROM (reprogrammable ROM store)
Some cards don't have this EEPROM (GPU maker cut cost and put file on PC to load it) These are hard (HW mod with soldering! haha) or can't be used.
I know very little here but just noting a couple of things related to this effort
GOP = General Output Protocol. I presume this is how GOP displays stuff during boot. Inserting Apple GOP into vBIOS and then flash vBIOS ito a PC card to be used in these iMacs is probably what is required. Like I said, I know very little here. @Ausdauersportler github dev link include final vBIOS and lots of parts + info on how to make one :)
VRAM is very complex GDDR memory. To go so fast, GDDR interface is quite complex. Besides electrical signaling technology like differential pairs, individual line drive adjustment to handle board trace length variations. They even include transmission error retry protocols (They dial up the speed so fast and accept a small % of error). Its probably impossible for the main memory vendor (Elpida, Samsung, Hynix) to make identical interfaces.
So you will see its important to find vBIOS to flash based on the VRAM version.
These are 2 different GPU to LCD communication signal standards. eDP = embedded Display Port LVDS = Low Voltage Differential Signalling. Without going into any details, just know GPU to LCD has a lot of data. Imagine a 2K or 4K pixel resolution display that is updated 60 times per section. These standards are designed to send these pixels at high speed to the display. eDP and LVDS cable and detailed transmission methods are different.
9,1 and 10,1 21" use LVDS
10,1 27", 11.x 12.x use eDP
Since some GPU cards can be used with both LVDS and eDP Macs, there are 2 different version of vBIOS for each of the LVDS and eDP version of the Mac. And of course there can be even more vBIOS versions depending VRAM versions
Note 10,1 (late 2009) nomenclature has both 21" and 27' models which is unusual. Furthermore, the 21" version is LVDS and 27" is eDP. The same Radeon HD 4670 card is in both of these systems but run different stock vBIOS due to LVDS and eDP. So when flashing cards for 10,1 make sure to choose LVDS and eDP correctly.
EG stands for Enable GOP. vBIOS with this feature will support iMac's original boot menu (press option key during boot). Without EG vBIOS, option key will do nothing and only choice is to boot Open Core boot menu. IF OC boot EFI was corrupted during installation (there are ways to get wrong settings including automatically by OCLP), then you are stuck because iMac boot order is CD->internal drive->USB. Can't boot OCLP installation USB boot stick while iMac is trying to boot a corruoted internal drive EFI and require removing internal drive to start repairing the software. SO whenever possible, look for EG version of the the vBIOS. However, EG isn't always available and making a rescue CD is a good idea (see below)
In general, it seems AMD vBIOS has both EG ( link ) and no EG ( link ) variants but nVidia Kepler vBIOS are all EG enabled.
WARNING : There are EG, EG1.2, and EG2 notations. I believe (don't have experience) EG2 require corresponding iMac logic board bootrom upgrade. Generally avoid unless you have expertise to do that.
As noted in above EG/EG2 section. None EG* VBIOS can require internal drive extraction (iMac LCD removal) if OC boot menu software is incorrect/damaged. See here on how to make a rescue CD
https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/post-33444384
Multiple ways. Easiest is probably GRML especially for end user. But it doesn't always work. See next.
NOTE 1 out of 3 iMacs I tried couldn't use this method ( link ). Next method is much more challenging using CH341A flashing USB dongle. Likely require soldering. Thus, this step should be done at the beginning to see if you are able to use the easier GPU flashing method.
Alternative is EEPROM flashing sticks (CH341A with EEPROM flashing boards and sockets) Likely have to modify dongle (soldering) to enable 3.3v programming (if EEPROM is 3.3v) link