I've only just realised something - obviously I'm playing through it again, and as it is so big I have kept it on my hard drive all this time. I have just noticed the goty edition in my GOG galaxy list. Is there actually any difference between the goty and the version I will be playing right now?... I'm pretty sure there isn't but thought I'd check!(the goty description does mention a few things such as better stability. I imagine the game was patched with that anyway?)
If you have the latest patch and expansions, there is no difference except the menu switches theme in the GOTY edition.