Apple has fixed the problem and all apps that were affected by the issue will show up in the App store with an "update" available. I know for certain Gun Log was affected because all of my devices show an update pending for Gun Log.
Please backup your data. This time the issue may have been Apple's mistake, the next time it could be mine.
Apple Says It Has Fixed Server Bug That Led to Crashing Apps
“We had a temporary issue that began yesterday with a server that generated DRM code for some apps being downloaded,” Apple said in a statement to AllThingsD.
If Gun Log 5.1 crashes on launch after the update, do not delete the app. Check and see if there is another update available. Also, if you have backed up the database, secure that backup. If you don't have a backup, and have not deleted the app to reinstall it, the data is still there, so do a backup as described below.
Here are some more links concerning the issue:
I apologize to the users that are effected by this situation. If you do not have backups of your data then the data is lost.