It won't really work for laptops, at least without a lot of research and fiddling. You need a desktop with at least one GPU, for the guest. If you want to run the host as GUI, instead of headless, then you need a GPU for the host as well. You can shut down each guest and boot up another one but generally you'll need a GPU for each guest you want to run with a GUI, if you want proper hardware acceleration and if you want to run them simultaneously. I run on a 4 GPU system for example, one for the host and 3 for each OS.

However, for certain Intel systems, there is something called GVT-g which allows sharing one GPU with both the host and guest, as you mentioned. You can try that and see if it works, I'm not sure if it does on a laptop. Or if you don't really need GPU acceleration, just make a VM as you normally would. I suspect since you don't say whether the notebook has a dedicated GPU that you don't game and thus wouldn't necessarily need GPU acceleration.

"for certain Intel systems" makes it sound rare. https://wiki.archlinux.org/index.php/Intel_GVT-g says, anything Broadwell and up should work, so any device from 2015+ should work for creating vGPUs, and thus be able to use native acceleration at host and vm-guest level!

Yes, but is that also the case for mobile Intel CPUs? I was unsure so I said "some" rather than "most."

GVT-g works on my mobile Intel GPU.

Do you, or anyone, happen to know, if WSL2 (=HyperV), or any other Hypervisor on Windows supports GVT-g (well), so that (wsl2) linux applications running on-guest can benefit from hardware assisted rendering?

WSL 2 does not support GPU pass through currently. I'm not sure if Hyper V does either, but this seems like it will change soon due to Windows 10 X. There has been development where for emulated Win32 apps they do use GPU / hardware acceleration. Sadly, with the the Neo being delayed past 2020, we might not see it until next year.

I wasn't aware Intel still made "Desktop-AddIn cards", or that the Non-Mobile versions are in any relevant way different to the mobile versions. In any case, the page I linked above does not seem to draw a distinction. So I assume, mobile versions are good to go too. Heard/read about differing experiences?

I have it setup on my Thinkpad T580, seems to work fine enough. I do get the occasional visual corruption with it but it works well enough for me to run things like Luminar and other photo editing things that need hardware acceleration. It doesn't seem to work reliably with video encoding, which might just be a driver issue somewhere that I haven't found a solution for yet.

In a World where virtualization has allowed to run several virtual machines on one physical machine (not even high-end) for 15+ years, it looks like a strange concept to need one physical piece of hardware per VM.

I actually understand why it makes sense, simplification and best performance for GPU intensive context, yet it feels like "let's virtualize, not really".

The real reason is money, as it usually is. Nvidia and AMD GPUs can in fact split their resources per VM - if only you buy their enterprise/research versions of their cards, like Quadro and Firepro. Most people have no need of VM GPU passthrough, so they don't support it for consumer cards. Intel does because they are not really in the graphics space (not yet anyway, we'll see how Xe does).

With Linux 4.16+ and an Intel Card of age Broadwell or newer (~2015+) you can use this howto: https://wiki.archlinux.org/index.php/Intel_GVT-g. Adapt from Arch Linux if you have something else.

I think you will need at least 2 GPUs (one for the host and one for the VM). Maybe get an additional eGPU.