How to unlock different game modes? Use the following tips to learn how to get them!
Casino Mode: you have to complete the game and choose "New Game" -> "Casino mode" which will be above "Normal mode".
Cherokee mode: complete the game on the Normal difficulty.
Concept art galleries: complete the game on the Normal difficulty setting and you will get the 1st gallery unlocked. Successfully complete the game on the Cherokee difficulty setting to unlock the second gallery.