That’s a transfer within the platform, very different from the scenarios I described. There is no method supported by GOG or Steam to transfer a game to a competing platform.
You can’t open a support case and tell them “sorry I actually wanted this game on GOG, can you transfer it to my account there?”. At best you could ask for a refund, obviously if you’ve played the game enough you wouldn’t even be able to ask for that.
StS2 was being developed in Unity iirc, and they moved to Godot for that new game (and possibly also future games?).