To unlock Gallery 1, beat Trial Mode without continuing. Or, play the game for more than five hours total.

To unlock Gallery 2, beat the game on any difficulty. Or, play the game for more than ten hours total.

To unlock Sound Test, beat the game on Easy without continuing. Or, play the game for more than fifteen hours total. Sound Test will be available in the Appendix menu.

To unlock the Game Mode option, beat the game on Normal without continuing. Or, play the game for more than twenty hours total. The option can be found in the Appendix menu.

To accumulate more Credits, simply play more! By the time you've logged four hours on the game, you'll have more Credits to begin with, making the game slightly easier to beat (oh, who am I kidding?).