I haven't heard anything about the bonus material coming to GOG, which is kind of frustrating seeing as it's not like there is a cost involved. You can try asking Remedy yourself here: https://twitter.com/remedygames

Hopefully it will show up in a week or two.

I'll do that, thank you.

Seems to me they're making last-ditch effort to push the sales on Steam. I'm positive it will be coming here at some point.

Hmmm, maybe I was wrong about it being free, at least - http://store.steampowered.com/app/108726/

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This one is free: http://store.steampowered.com/app/268560/ . And it seems to have more material (3 gigabytes) than the paid one but doesn't have the "Alan Wake File" PDF. Both DLCs have soundtracks, and they are not the same.