If I get a physical copy from, say, amazon do I get the extra wallpapers and avatars and stuff advertised as a gog exclusive? I want a physical copy and I want the extra stuff so since the physical copy is just a gog code in the box I was hoping it would come with the extra stuff.

It is nothing in-game related the stuff in the gog.com purchase lot, so just wallpapers and such it seems and a digital booklet and more than lightly it will get shared by others after launch if you really want that kind of digital stuff.