WebGPU doesn't offer any facilities relevant to GUI development except a method of putting pixels on the screen.

If you're suggesting WebGPU, you're really suggesting a drawing technology, not a GUI toolkit.

No I didn't mean to imply its utilization for GUI development. I meant for it as an example of cross-platform technology that maintains good performance while being extremely cross-platform

Plenty of examples of that. I could name dozens outside the web space.

And will it look and act like a native app? Will it support all of Apple’s built in accessibility affordances that a developer can get for free by using Apple’s native toolkits (and Microsoft’s)?

a11y is definitely a critical and often overlooked issue. And this is one area where I find myself in 110% agreement with the title of TFA: Stop Building Closed Ecosystems.

If the Apple and MS a11y frameworks were not so distinct from one another, wrapping them in a cross-platform layer would be much easier. But they couldn't be bothered, largely because they both have enough users whose priorities match your own: are the apps on the platform good enough for them. The incentives to standardize a11y at the API level across OS platforms is a tiny fraction of the incentive to standardize e.g. WebGPU.