DGroves has much more experience with the HP Thunderbolt cards than I do. I got a HP TB3 kit off eBay that looked brand new with all the short DP cables, GPIO cable, and untouched sealed card, but was not in HP shipping box. So far have not been able to get it to work in my Z4 G4 and I may beg him for help in another post. I can't see it in Device Manager (present or missing) but latest software released last year was installed correctly (with card attached to proper PCIe slot and GPIO cable attached), and the software can't see it either. I fear the card might not have firmware on it.
The Z4, 6, 8 G4 workstations all use the same TB3 kit based on their respective HP QuickSpecs so the GPIO cable would be the same.
He and I worked on some TB3 GPIO pinout stuff on the post HERE and my scribbles and pics are on that page. So far TB3 has been a time sink with little reward.