If a game available on GoG.com was never released for Windows before, can Windows be added as a platform?
Certainly that's the kind of loophole I'm employing to cram in unsupported platform-independent games as "browser" games.
When you buy it and it is supposed to run on a Windows platform, for instance through DOSBox provided along with the game, then yes.
If a Windows-specific version of ScummVM is included, then count it as Windows. If it's just the data -- much to our detriment, we don't currently document platform-independent data yet 8)