Are some games windows 10 only?

So they don't ever work on 7 or 8.1?

Is there a list of windows 10 only games?

Not sure there are any windows 10 only games. 32bit v 64bit can be an issue. But otherwise should be ok. Now, if a game has old drm like securom then that won’t work on win 10 as it was disabled, should t affect GOG games.
Look at the list of valid operating systems on the game page, if it says supported then you get support, otherwise it may well still work but you will not get support.
Win 10 is very compatible, as is win 7, win 8.x I would avoid.

Thanks for the answer.

Only one I'm aware of that's actually Win10-only is Six Ages: Run Like The Wind.

In other supposedly Win 10-only cases like the Panzer Dragoon: Remake, Cyberpunk 2077 or Iron Harvest it's just preliminary for now.
Come release they're likely to run on Win 7 and 8 just as well (Iron Harvest, for instance actually does run on Win 7, as confirmed by the devs) but since official support for Win 7 has ended earlier this year it's basically a legal safety measure to not explicitly list it as a supported OS (devs could be held accountable/liable if it turns out to not work on Win 7) until close to the actual release.

There's no list that I know of.

With only a very few exceptions like Six Ages: Ride Like the Wind), "W10 only" is mostly equated with "DirectX12 only" games as W7 does run nearly all DX11, OpenGL & Vulkan stuff. And the only people trying to artificially lock W7 out like that are Microsoft (for obvious reasons). Eg, Quantum Break was W10 only via the Windows Store. After it sold badly enough that Microsoft re-released it on Steam, it magically became DX11 / W7 compatible... ;-)

Cyberpunk 2077 is still unknown until it's actually tested ("W7 not officially supported" and "Tested & confirmed to not run under W7" are not the same thing). It may run, just be that CDPR won't support it post-Microsoft end-date support.

Any DirectX12 only title is supposed to only run in Win10, however Microsoft added DX12 support for Win7, allowing some games to run.

Most DX12-only games should only run in Win10, like Forza series and Gears of War series.

Probably there is no game on GOG that's Win10 only.