I was under the impression that GOG games came patched and updated to run on new OS. You are saying I need to patch it. What are the required patches and will it mean I have to restart the game?
GOG games do come patched and are optimized to run on modern OSes. Still, rarely, you come across one like Arcanum, which was already very glitchy to begin with, and there's not much GOG can do, using only the official fixes and patches, to make it glitch-free.
The first post in this thread gives you detailed explanations on the essential mods and how to use them, in case you're interested.
I'm not a mod fan, myself, so the only one I installed was the one I linked you in my previous post (you can find it in the first post of the thread, as well), in order to make my copy of Arcanum more playable. If you choose to install that patch -- and you should --, there's no need to install the game again, though it's likely you'll have to start it all over, as unpatched Arcanum save games tend to be incompatible with the patched game.
As for "required" patches, that's about it. Other ones increase resolution, add in-game content, etc, but the only one you really *need* is the universal patch.