All GOG releases are patched to the most recent version.
With Night of the Raven installed (which is the default) Gothic II has one noteworthy bug which has never been fixed in any release: when talking to Lares you get the option of asking him to take you to a farm; if you do this before having him take you on a crucial quest-related journey the dialog option will disappear and you will be unable to proceed. It also locks out an optional but useful journey option later on. For best results, don't bother having him take you to the farm at all and find it on your own (which is really easy).