The FPGA he is using has native pcie so usually all you get on this front is an interface to a vendor proprietary ip block. The state of open interfaces in FPGA land is abysmal. I think the best I’ve seen fully open source is a gigabit MAC
There is an open-source DisplayPort transmitter [1] that apparently supports multiple 2.7 Gbps lanes (albeit using family-specific SERDES/differential transceiver blocks, but I doubt that's avoidable at these speeds). This isn't PCIe, but it's also surprisingly close to PCIe 1.0 (2.5 Gbps/lane, and IIRC they use the same 8b/10b code and scrambling algorithm).
Thank you very much for the references. These look like great projects and I am happy to see that I’m a bit out of date. The vendors don’t appear to be making anything easier though; it appears these projects are still supporting devices by making the brute force effort to build the abstractions to vendor specific stuff themselves.