Can GOG version of the game use save file from original Sacred 2 Gold?

You should be able to. The initial S2 was developed with multiple patches and version to the final one.

P.S. - ALWAYS back up your files in case one of your characters falls in battle. They lose a Survival bonus which usually means enemies are tougher and give a bit better loot.

Yes, you can. However, with the patch included now in the base game, there could be some changes which could cause certain quests to be bugged. It would be better to start a new game. Somehow, starting a new game is a good idea if you haven't played Sacred 2 Gold in a long while. There are also some interesting mods you might like.