As you may know, DUSK HD comes as a separate download available in the "Extras" section. There is no dedicated GOG page for DUSK HD. However, if you want to you can get DUSK HD to run via the GOG Galaxy application. All you have to do is install the classic version, download the HD update from the "Extras", unzip it and then copy all files to the folder with the classic version. Answer "yes" when asked about replacing the existing files and you're good to go. From now on when you launch the game via GOG Galaxy it will run DUSK HD.

IMPORTANT STUFF:
1. Since you have to overwrite the existing files, the only way to play the classic Dusk again is to completely uninstall the game and reinstall it.
2. When you play DUSK HD with GOG Galaxy the time total gets properly recorded to your profile, but you CANNOT obtain any achievements. These have to be obtained in the classic version.