Guide to unlock all gallery:
At main menu, choose the last option, then choose unlock all Gallery.
It will require you to put a 6 digit password.
If you go to files explorer and into the game folder, tap on the file that is called ※回想全開放スイッチ※.txt
You will find a 6 digit code within this text file. I am unsure if this is different between downloads. But here was my one below:
6731031