Finding video drivers for macOS
You cannot just use any graphics card with a Mac, unfortunately. You will need to have both driver support as well as a specific Mac firmware on the graphics card itself.

It is possible to buy non-Mac graphics cards and flash the needed firmware onto the card. It only works with specific models though. Apart from that you'll need to buy the "Mac-version" of the graphics card to get the right firmware. The selection is very limited compared to what you're used to on Linux.

In regards to drivers - if you choose an AMD-based graphics card, the driver is almost always included with macOS. If you choose an Nvidia-based graphics card, you can download the driver from their web page.

You can also find a good driver listing for Nvidia cards here:

http://www.macvidcards.com/drivers.html

The same web site also offer Mac graphics card for sale, as well as listing which ones are compatible with each model of Mac Pro.

The official Apple list of supported graphics card for your Mac Pro is available here:

https://support.apple.com/hr-hr/HT201805

Note that there are options out there that aren't officially recognized by Apple, but work well anyways.