I can't understand why the developers have removed the OpenVR implementation from the game.
There is no need from Steam to use OpenVR. MAny games in GOG use it, without problems.

Please, enable VR in GOG version.

Yes, please bring it back.

While SteamVR is their prioprietiary implementation, their standardized it as OpenVR.

Previous comment about SteamVR(proprietary implementation of OpenVR) is still valid
You can link to OpenVR (not OSVR!), and it will work with SteamVR when available.

But theres now also OpenXR. 1st implementations were Monado(Linux) and Windows VR/MR:
https://www.khronos.org/openxr/
https://en.wikipedia.org/wiki/OpenXR
https://docs.microsoft.com/en-us/windows/mixed-reality/develop/native/openxr