Knallerbse: I know I must be stupid, but I can't find the extras. (Not the preorder extras, but the extras that come with the digital premium edition.)
Please give me a hint!
When you click on the game on the GoG site you see the list of install files and the GoG installer link in green. That little pop up window actually scrolls and your digital premium extras are available below to download.
*Don't feel dumb, I missed it too ;)