Sorry for causing the trouble.
To who got the issue about the photo files saved on SD card are not completely displayed in Gallery after recent system update, you are able to refer to the situation you might encounter below to get the possible solutions. We are afraid that not all the suggestions can work for your cases. In case they are not, we still highly suggest you contact ASUS ZenTalk at http://www.asus.com/zentalk/forum.php?gid=36 to get further help.
When the photo files on SD card can be still correctly displayed in File Manager but not Gallery or Google Photos, please do try the following suggested methods which we collected from other users who solved the problem by themselves.
(1) Unplug/re-plug the SD card
(2) Rename one photo file name or an album name on SD card.
(3) Restart the system.
If (1)~(3) can’t work, it’s possible that there are some problematic files or bad sectors available on the SD card to stop the media engine working normally. If that’s the case, we suggest you unplug the SD card and plug it with your PC. Copy all the files for backup from SD card to your PC. Then re-format SD card and copy the backup files back to SD card again. After that, you may re-plug the SD card on mobile device and then check if all the photo files can be seen then.