Somebody else with this problem?

I asked the support

Same.

No offline installers for the free items/skins

Supposedly it's already integrated into the game:
"The DLCs are present in the game (please do not worry that they are missing from GOG GALAXY - this is as intended) and I have just rechecked this with our QA department. There has indeed been an issue with the achievements which should now be resolved.
If the DLCs are still not present in your game or you are experiencing difficulties with your Achievements please get in touch with us directly and I will be more than happy to help."
https://www.gog.com/forum/general/release_xuanyuan_sword_vii_664f9/post42

Thank you!

Missed the other thread.