I cant find that in any of the configs. Can you be a bit more specific?
It's not in any config in Galaxy, perhaps due to GOG mistake or some miscommunication with developer. You must run it manually:
Win+R, type cmd.exe, confirm
cd "C:\GOG Games\The Solus Project" // assuming that's where you installed it, adjust as necessary
TheSolusProject.exe -solusVR // again may need some adjustment, not completely sure about the .exe name
Or you could make shortcut to the .exe and add the parameter there, for a more permanent clickable solution.
edit: darned gog forum eating my extra spaces, we need [pre] forum tag...