Maybe there was a change to the soundtrack or other extras included with the game?
I don't have the GOG version installed, so can not check for recent changes (already had 4 different versions installed).
If you cancel, then select the game in Galaxy, the More button should be blue, with a 'update available' message, and then a link underneath showing the version names, which should bring up a changelog.
(or More, Settings, uncheck automatic updates and look at the list of versions available; searching for the version number given may turn up a change list)