As the first troubleshooting step, we usually recommend to do a "clean install" of the application.
Please follow the steps below and if it doesn't help, contact us via this support form so we can investigate further :)

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- First remove the GOG GALAXY app from the Applications directory.

- Then please also remove the contents of these directories:
-- /Users/Shared/GOG.com/Galaxy
-- $HOME/Library/Application Support/GOG.com/Galaxy/Configuration

You can open it using the "Go to Folder" command accessible with the Command+Shift+G key combination or selecting Go > Go to Folder from the Finder menu.

- Now restart your Mac, download the current installer from:
https://www.gog.com/galaxy
and reinstall the GOG GALAXY app.