Just got this game on GOG since it is DRM-free, but I cannot find how to launch the game in VR (Oculus Rift S). Really frustrating with the lack of instruction on VR since the support is listed in the description.

Assuming you do have Oculus Home installed, could you please attach your se.log file so we can see if there are clues about this issue? You can find it in the SpaceEngine system subfolder.

I have the HP G2, which worked with the Steam version, but like the OP... the game on the GOG edition shows headset option blacked off so not detected despite having the Windows Mixed Reality tool running

After a bit of digging in the config files I found the solution, which forces the SteamVR app to run (without Steam running) and allowed compatibility for my headset, but sure the solution will be universal.

The solution is thus:-

Goto the subfolder of the game

GOG Galaxy\Games\SpaceEngine\config\ and edit the file called main-user.cfg

Scroll down until you see this line and in my case set to 2 to force Steam VR detection mode, now the headset can be toggled manually in-game display menus and ticking VR Headset on and off.... which worked, even the handsets detected correctly :)

// VR settings
VR
{
VRRuntime 2 // used runtime for VR: 0 - auto, 1 - force Oculus runtime, 2 - force OpenVR/SteamVR runtime