I use many different computers (you cant take your laptop with you in some places) and have most of my games on an external drive.

Is there a possibility to make the gog version portable? The save is always stored locally in the user directory.

Only the developers can do that. GOG can only distribute what the developers provide -- they may not alter it, other than the approved installation package.

If you use the Linux version, it is easy to write a short script to change the saves/settings location to a place on your external drive. I could even write it for you if you’re interested.

If you use the OS X or Windows version, it is probably quite easy too, the difference being I won’t be able to write it for you.