So I originally thought it could have been a blank VBIOS issue. But you're telling me that with a GOP ROM it wasnt working? You won't see LED 3 with a GOP ROM because be there is no Firmware initialization/handshake with the iMac's EFI, it would have to be EG2 modded for this to work. I know it's probably to late, but I'd like for you to create a High Sierra install with the login screen Disabled, plugged into Ethernet, and remote screen sharing enabled. Then you can see from another Mac if the card is recognized.
But more than likely something is slightly different about the card because it's a HP model and it's not recognized.
I have pics of
@internetzel card which did work, but had a blank VBIOS on arrival, you can use these pics to compare to you HP card to come to your own conclusions.
Also please use punctuation it's late at night, I can barely read.