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 received she began work on the latter.

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.
 * Robin wasn't born with her Anti-Akuma Weapon on her arm; it was surgically implanted onto her body, the arm having originally belonged to Cross.
 * Lenalee is not member of the Black Order but instead a waitress at a Chinese restaurant 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.