Due to a minor bug, previously the Collector achievement for completing the ending gallery was failing to unlock when you achieved your final ending and would only unlock the next time you finished the game after that. This should now be corrected.
If you have previously completed the gallery but not received the achievement, you will still need to reach an ending again in order to trigger the check and get the achievement. Any ending will do, even a bad one.
Please let us know if you have any problems or if the achievement still fails to fire.