Shortly after installing GOG Galaxy (around the time Galaxy 2.0 was released IIRC) I had also set up Steam integration in the Galaxy client, and back then the synchronization did work (all games I had in my Steam library then are listed in Galaxy as well). But at some point the synchronization stopped working and new games from my Steam library don't show up in Galaxy anymore.
Deleting and re-creating the integration doesn't seem to fix the issue. Is the synchronization just terminally broken?