Actually, there's a temporary workaround which you can use and works till you update to a new version (works with any app or package not signed with an Apple ID)
Simply right-click open and you'll not see the error message.
No need to disable gatekeeper in System Preferences
That's correct, or users could use the right-click workaround.
Most users, however, won't know this workaround and it shouldn't be necessary to lessen the security of OS X just to use (or update) LibreOffice.
I'm sure the Document Foundation could afford the necessary app-signing.
Apple implemented app signing by default in 10.8. Users can either turn this off in System Preferences (removing the error), or the LibreOffice team can pay the $100 annual fee to get a properly signed app.