So there's a slight problem with the gallery, unlocking the naked version of the girls is quite troublesome, unlike the normal clothing which you can buy on the store, finding the "naked clothing item" is not easy and unless you find it in the game you can't see it in the gallery, I finished the game with 3 stars on every mission on both normal and hard difficulty and still I only managed to unlock the naked succubus.
Tried messing with the save file but it's to much complex, if anyone manages to unlock everything please share it with us!