Shamelessly copied from https://forum.proxmox.com/threads/gpu-passthrough-hp-elitedesk-800-35w-g2-core-i5-vpro-6500t.135888/ with some adjustments and reported for posterity/self history.
First make sure all virtualization stuff, includint VT-d are enabled in the BIOS
Install ProxMox, then login into the console as root
vi /etc/default/grub