You should have the 3.0 version of the soundtrack on your GOG account (files divinity_original_sin_2_ost_flac_3_0.zip and divinity_original_sin_2_ost_wav_3_0.zip)... that includes the 7 audio files mentioned in that Steam announcement. I just checked, and I have them.
The High-Res versions of the artbook and lorebook are still not available on GOG, though. If you backed the game through Kickstarter, you can find those files on the larianvault. If not, I fear you'll have to complain to GOG support about this...
