GOG Galaxy seems to not be able to find DLCs that have been installed using the offline installers.

For example I installed Prey and the DLC Mooncrash using the offline installers but when I open the "Manage installation > Configure" menu the DLC is always not active. When I click on the box to activate the DLC Galaxy starts to completely redownload the whole DLC instead of just scanning the game folder for the DLC files. Please fix this.

This still happens today.

Prey, XCOM2, Deus Ex Mankind Divided, etc... other games have shown the same behavior. The worst part though, is that asking Galaxy to verify/repair the installation of these games causes it to redownload the entire DLC. For XCOM2 - War of the Chosen, that is a whooping 37GB redownload...

You can use the tips in this thread to work around it for now.

Make sure you report the issue in Galaxy, more reports might nudge them into actually fixing it.

Thanks for the tip. Will see how it goes.