I just downloaded Icewind Dale, and I'm wondering if a) the GOG version is fully patched and b) if there are any unofficial patches I should definitely use.

http://www.gog.com/en/forum/icewind_dale/mod_guide/post1
The tweakpack contains some fixes, so I'd recommend using at least that. You can pick which components to install, so you could just stick to the fixes, should you want an authentic experience or something.

All of GOG's games are patched to the latest available official version at the time of the GOG release, so you don't need to hunt down any official patches. Check the mod guide linked by Lord Cinnamon for more unofficial fixes and tweaks.