Another big cheers for the continued generosity, +1s all around.
You can ask GOG via support to change those into gift codes: https://www.gog.com/support/contact
->my orders...... ->Problem type: order change .......
Shall even work with those added games
If the last sale is anything to go by, GOG will probably not change XP games to gift codes. They can still try, but I wouldn't hold my hopes high, if in their place.