I didn't see any mention to this, so:
Is it possible to access old version of the game on GOG (vanilla Driftmoon, as they say) as one can do on steam (https://steamcommunity.com/app/263380/eventcomments/2260188150856341730) ?

No, it’s not. You can only access the one version presented. You can vote for it here:
https://www.gog.com/forum/general/provide_a_full_and_complete_changelogged_download_system

Only if you owned the vanilla version and downloaded it already, prior to this enhaned version.
The old version is not available anymore under the new (and now solely available) version.

Edit: maybe I should clarify - I don't know if there's a possibilty to choose between versions in-game.
I haven't downloaded this new version yet.

I bought this game back when it was new here and still haven't touched it for some reason but i guess it's time to change that.

As for the original verson, is this enhanced edition worse or better for some reason ? if the original is not better then i guess i'm content with the EE only.

You can choose old versions in Galaxy, but I think a better way would be for GOG to include an installer for the last Vanilla version in the goodies. You've misnamed it, not only on the news post but in the store. It's ENCHANTED Edition, not Enhanced. You've even got a contradiction between the text on the store page and the logo on the store page.

Would appreciate an option to download the old "Driftmoon" too.

Given that they already provide old versions through Galaxy they really should do the same for the installers.

Also GOG failed to mention that the Enchanted Edition now supports Linux and Mac. The old version was Windows only.

What's new compared to the vanilla Driftmoon?

-Driftmoon now runs on Windows, Mac & Linux!
-We've added the Mystery Box, full of surprises for both you and your friends!
-New characters, quests, items and skills (through the Mystery Box and elsewhere too)
-Major graphics update: better textures to look better on modern screens, lightmapped shadows, and the lot.
-Updated user interface: easier to use, looks better, and scales better: from your smallest screen to your living room TV. (still including the possibility to adjust text size, etc)
-Tons of bugfixes, dialogue fixes, quest fixes and improvements.
-Improved music, with an extra melody. All music by Gareth Meek - Thank you again, Gareth!
-Gameplay enhancements: quest reminders, Silver feather counter (very useful!), and more.
-The possibility to recover from a broken save (for example if your disk was full).
-And more surprises! Because there's bound to be something cool that we forgot to mention here

Always good to see more platform support with the addition of Linux and Mac.

Thanks but i rather meant if there's some hidden issues like if it's really buggy, runs poorly, gameplay enhancements are for the worse etc as people seem to want original too.

If this version truly is the superior one than i personally don't see the value in having the old ones around too and will be just as happy with this version.

To Ville and Anne, thank you for the ongoing support after all this time and providing all these updates free of charge to previous buyers! :)

Ah, I see.
And I agree: newer and better is often preferable...if they don't mess things up through their improvements (to which I can't say anything in this case - I haven't downloaded/tried the new version).

All I can say is: back when I played Driftmoon (vanilla) I didn't encounter any bugs.
Of course, that isn't saying much, so take that with a grain of salt. ;)

Thanks for adding Linux support!