Found a workaround!
You'll probably still have to clear out the GOG cloud saves of the old data, but a user on the GOG forums
here has a solution.
Don't delete the individual campaign folders entirely, just delete the
contents, while leaving the campaign folders and making sure they're Read-Only (they probably are by default).
This worked for me; the free campaigns did not show up when I reloaded the game and my sync files stayed small enough to properly sync with the cloud.
