This isn't your normal post, about whether an NVIDIA card is supported or not. This is a post about a system using one of the old Kepler Quadro K600 cards.
I have recently removed a the dGPU from one of my HP server Hacks. As I had to play merry-go-round with a few GPU's after I replaced a defective dGPU in my son's PC with one of my PowerColor RX580 cards.
I installed a spare (second-hand cost £10 on eBay) Quadro K600 1GB dGPU in to the HP ML10 Gen9 server, this is a Skylake Xeon system, P530 IGPU (Headless framebuffer), using iMac17,1 SMBIOS, running the latest version of macOS Catalina.
The Quadro K600 works just fine, graphics are good enough for general computing, media serving etc. I screen share when managing and maintaining the server, but have a spare Dell display (1920x1080) connected via DP, and it works perfectly well with both connections.
Here are a few screenshots from the About This Mac and System Information regarding the Server and K600 dGPU.
Server has 2x PCIe X8 & 2x PCIe x4 Slots
Even shows up in the PCI section.
These Quadro K600 dGPU's can still be obtained second-hand on eBay for between £15-£20 in the UK.
Just remember if you purchase a second-hand dGPU like the Quadro K600 that the thermal paste is probably long past its expected usefulness and that you will need to clean the heatsink, clean the fan and replace the paste for the dGPU to run at decent temperatures.
HWMonitoringSMC2 app screenshot attached below, showing some more information.