What is the best place to ask for a sequel of a game?
Contact the publishers of said game and ask very politely.
Contacted MangaGamer, but they said it's very unlikely they publish this VN on GOG, and advised me to buy the game on Steam or MangaGamer. Well - MangaGamer has no PayPal or iDeal, so I am forced to buy it on that Steam platform. I can only hope the VN runs without Steam client, so I can back it up to run it without the need to have a client running in the background. Still - It makes me sad...
Anyway - Thanks for the advise. I replied here so other people will know it make no sense to wait for the VN to appear here on GOG. It won't, so that's that.