I know there were limited edition addons for some of the bioware D&D games. Are those included in the GOG versions?

I can't remember exactly, but I'm pretty sure they're in there. At least, Deirdre (the girl in the back selling some really awesome items from PST; she was from the Collector's Edition disc) is there in the Adventure Mart. I don't remember if I saw Joluv in the Copper Coronet (he sells IWD items, and was from the GS/EBG Preorder bonus disc).

If I'm not mistaken, Dierdre and Joluv were also added in a patch, so that everyone could have access to it. I know my disk copy of the game had both, and I know I didn't have the collector's edition.

If you can't find them in the game you can donwload the bonus content from here: Bonus & Collector's Edition CD Merchants

Deirdre was added in the ToB patch. Joluv wasn't. If you are missing Joluv, you're better off using the "Bonus merchants" component of the G3 BG2 Tweak Pack, which can be easily uninstalled (unlike SP's pre-WeiDU override dump stuff).