In order to obtain the gaiden chapters in the game you must fulfill certain requirements.

Chapter 5x: Use a door key to unlock the door to the empty room in chapter 5. Once it is unlocked, have any character step inside the room to obtain a Talisman. Upon finishing the chapter, the player will be met with the option to go to chapter 5x, a sidequest.

Chapter 7x: Beat chapter 7 in 15 turns or less. At the end of the chapter, the player will have the option to go to chapter 7x.