Did you check the tick box for "Show with DLCs and extras"? See the attached image.

As I said in your other thread:

On the site (i.e. in a web browser), if you use the search box at the top (magnifying glass), that will show DLC as well as base games. However on the Browse All Games page, you must check Show with DLCs and extras in the sidebar.

Inside GOG Galaxy, the search bar (magnifying glass) annoyingly searches only other stores. To get to the GOG store, click Store in the Deals category in the sidebar. From there, click More offers at the top near the center. That gets you the aforementioned Browse All Games page on the site.
Inside GOG Galaxy, you can also click on a game in the Owned games section. From there, click the "More" button (the one with two horizontal sliders next to the Play or Install button) and choose View in store. That gets you the store page for the game, where DLC is usually listed on the right side.
To double-check that nothing's missing, click the link with the developer name. Then on the search page that opens, check both Show with DLCs and extras and Hide all owned products.