All I can download from GOG.com is the Linux version of Northgard. I need the Windows version to be
downloadable. Can you fix this

Sorry I now know how to download the windows version. Thanks

When you are in your library, click on the icon of the game and you will see the downloads. Just below the name of the game (upper left corner) is the System: drop-down. The default is Linux, so just click Linux and then choose Windows. Now when you download the contents, they will be files for Windows. The default is probably due to the system options being sorted alphabetically (Linux Mac Windows).