TAGma: In short: yes, you can install and play Skyrim offline, on any platform.
Details:
Skyrim does not require internet. The Steam version has a 100% optional menu for download community mods (called BethesdaNet/Creation Club).
In the GOG version, this feature is not present (you still can use third-party sites to download mods, such as NexusMods).
Another difference is that on Steam version, updates happen automatically, and the user can optionally rollback (which can be a confusing process for non-tech-savvy users.).
GOG Galaxy will notify you if there are new updates available, but you can ignore them and continue playing without installing it.
Actually, GOG Galaxy is not necessary at all, you can launch the game directly from the executable shortcut.
Thank you for the response. I appreciate the info and will finally buy this great game.