After defeating the final boss and triggering the good ending (Mary Reverser needed. If you still need to know how to get it, have a look at this guide: Ultimate Prison Break Guide from kichithewolf), you will unlock the bonus features in the main menu, where the gallery is also located.

There you will find all CG scenes from previously unlocked affection events as well as those of the heroines in all her job outfits, of which only a single one will be unlocked.

Affection level determines which heroine will be chosen for the good ending.

First of you will still need a save file before the final boss.

Then you will either have to rise the affection level of the girl, you want to unlock the ending for or if you already have all affection levels maxed out, just need to change party order, as the game choses the girl in the first party slot for the ending, if all affection levels are the same.

So either return to base and rise the affection of an individual girl and go back or just reload and switch positions before fighting the boss over and over to unlock all of the bonus galleries.

So not much of a trick needed here, but can easily be missed if you don't know how the game determines the girl for the ending or you don't keep a save file before the fight.

Enjoy playing the game and don't let the jail's mimicry fool you