Zone

is a one shot manga written by Katsura Hoshino that introduced the main concepts of D. Gray Man. Due to the positive reception it recived she began work on the later.

Differences
There are several differences between Zone and D.Gray-Man:
 * Akuma are created by one signing a contract instead of reviving the dead.
 * The main character was not Allen but a similar character named Robin who instead of being cursed by an Akuma was an Akuma herself converted by Cross by having Innocence grafted to her.
 * Lenalee is not member of the Black Order but instead a waitress at a chinese resturant in Japan, and is half Japanese.
 * The Earl hasn't taken control of Japan and it is free from his influence for the most part.