Crosmando: As a general rule games on GOG are always patched to the latest version, otherwise it would be pretty slack of them to be a digital service and expect us to patch the games we buy.
Coelocanth: Don't know about P?C, but the Baldur's Gate series is patched to the latest official patch.
Well, I wasn't meaning to suggest any slackness on GOG's part! ;-)
Maybe it's not always clear (to me, at least) what's an official patch and what's a recommended modification that's generally believed to aid playabiility. Some of them, I realise, are modification of the game to make it function better on a modern OS; well, I'm running Windows 7, so I figure I'm pretty squarely in the sights of the work GOG does to make the games stable in current environments. That's why I don't want to mess up any of GOG's modifications.
But with other mods and patches ... unless you're familiar with the game, it's hard to know what is needed and what is not.