The only games that can be added to your GOG library from outside sources are the retail copies of
The Witcher and
The Witcher 2 because those games are made by
CD Projekt Red who is the parent company of
GOG.com. The GOG library is not meant to be a list of all of your games, it is merely a list of all of the games you have purchased through GOG directly. So, in this case, if you wanted to get
Sacred Gold to show up in your GOG library, you would have to purchase it here.
As for your second question, the developer of
Sacred Gold is indeed
ASCARON Entertainment with the publisher being listed as
Strategy First. You can see all of the game's details on the
game's info page.
Lastly, welcome to GOG! I hope you enjoy your time here!
