How do I play newly purchased Zork offline

Download. Install. Play.

You don't even need Galaxy for that. Just download the offline 'backup' installers.

Sorry, don't know where to find 'backup' installers. I am in the dark about most of this. Last played zork w/ floppy and no other gaming and little computer experience. Many thanks for help!

Click on the Zork tab in the Galaxy client, then click on More, then Backups & Goodies. Click on the file and it will be downloaded. Exit the Galaxy client (closing it will NOT exit the client), navigate to your GOG folder and the backup installer should be in the folder under Zork.

In Galaxy you will first need to go to the Library tab to find the game.

And when browsing the store through a Web Browser, you will need to go to the library , click on the game thumbnail and from the menu that opens click on the "classic offline backup installer' or something similar at the bottom of that menu and download the installer. After the installer has been downloaded, run and install the game and play it. GOG games being DRM-Free you do not need to use the Galaxy client to play the game, it is optional. Also no internet connection required except for to download.

many thanks to all for instruction. I'm not sure I understand it all and it will take time to muddle through it but I'll get there!
I'm bad with sites I am not familiar with. it will be good to be able to refer back to these replies. thank-you for taking the time. Look forward to playing zork after so long....

If you use a browser: First you go to your account. Point with the mouse to your name and then select games. (see attached picture) Then you select ZORK from the games in your account (I used ZORK GRAND INQUISITOR as an example). That opens the game-tab and on the lower left side you find the 'offline backup installers'. Just download these, execute the install program and after that you can start ZORK like any other program through the start menu. Oh and if you don't want to download via browser because you have a slow internet connection, you can use either Galaxy (I don't, so don't ask me for help there) or any downloader to get those files. For example the old GOG downloader, which still works:
https://www.gog.com/downloader

Danke für den Hinweis! :-)

Yeah and for that helpfull advice I got downvoted! *grins*

Probably because you used German. ... Or because the trolls were bored.