If the game is large, it may consist of an .exe file and multiple .bin parts; you must download and keep them in the same folder for the installer to work. Via the GOG Website
GOG Galaxy provides two primary methods for backing up your games: downloading dedicated or manually copying installed game folders. Because GOG games are DRM-free , these backups can be stored on external drives and used to reinstall games without an internet connection or the GOG Galaxy client itself. How to Download Offline Backup Installers gog galaxy download backup
GOG's offline installers are self-contained executable files that include everything needed to run a game. Via GOG Galaxy and select the game from your library. If the game is large, it may consist of an
Under the section, click on the game’s name to start the download. How to Download Offline Backup Installers GOG's offline
Click the button (located at the top, near the "Play" or "Install" button).