I regularly do remote VNC and X11 access on stuff like raspberry pi zero and in these cases GPU does not work, you won't be able to open a GL context at all. Also whenever i upadte my kernel on archlinux i'm not able to open a gl context until i reboot, so I really need apps that don't need a gpu context just to show stuff
You can also trick any HDMI output to believe it's connected to a monitor.
One commercial product is:
https://eshop.macsales.com/item/NewerTech/ADP4KHEAD/
But I seem to recall there are dirt cheap hacks to do same. I may be conflating it with "resister jammed into DVI port" which worked back in the VGA and DVI days. Memory unlocked - did this to an old Mac Mini in a closet for some reason.
For the Pi Zero you can force a headless HDMI output in the config and then use that instead of a virtual display to get working GPU with VNC.