Similar 19th-early 20th century setting. Both worlds have some form of special ability (alchemy in FMA and Innocence in DGM) used by the main characters. Both main protagonists are 15-year-olds who have a strange arm. Both also have a group of villains with secret motives.
Both main characters have a master. They also joined an organization that hunt demons or homunculi.
Both take place in the same time period (end of 19th century/beginning of 20th century), have main characters that have lost their arm and have it replaced with a weapon who have had tragical situations in their childhoods. Allen (DGM) and Edward (FMA) are called "short". There are organizations in both anime (Excorcist for DGM, state alchemist for FMA). Both have a special enemy group (Noah for DGM, homonculi for FMA).
Both action-packed shows have dark plots with a fair amount of comic relief and easily appeal to a wide audience. The main characters (Edward Elric and Allen Walker) have several similarities as well, a few being: their unusual arms are both gained through traumatic experiences and provide them with means to fight, they are both part of special organizations, and they are both orphaned with painfully troublesome childhoods, as well as other minor aspects of their personalities. Both shows have a wide variety of characters, each with their own unique sides and backgrounds.
Both animes are similar in how each episode is often different and is one of the main characters' "cases". They keep you watching and are easy to watch even though they're both really long. Both animes have a group of people (Exorcists in D.Gray-man, Alchemists in Brotherhood) fighting some strong common enemy of the world and protecting humans from their harm. I highly recommend both of them and am sure that if you liked one, you'll like the other also.
Fullmetal Alchemist Brotherhood - D.Gray-man Similarities
Edward - Allen Similarities
About the same age
Over-reacts when called short
Both have an Altered Arm
Both cry when they can't save someone
Black Order/ Military
Regular Soldiers/ Finders
Philosopher's Stone/ Heart Innocence
These are some of the many things that make these anime similar
My Opinion I strongly Recommend
Characters are basically mirrored in both anime tons of action where nothing ever goes boring, Always traveling from place to place helping others.
Both the series center around a conspiracy group trying to gain control and change the world,with a protagonist that's part of the military working with his friends to defeat them. They're both amazing story's that keep you glued to the screen.
Both the main characters in these shows did something in their past of which caused something to change to them/in their life, and regret it. Setting out on an adventure to fight against an enemy whose plot is to do something sinister to the world for their own goals.
Great story line. Main characters have tragic pasts and they overcome their obstacles with gusto. Even the side-characters are well-developed. Major plot twists here and there. Once you start watching, you will get hooked and can't stop until you finish the whole damn thing.
Both D.Gray-man and FMA: Brotherhood last a little longer than the average anime show, without getting too long-spun. They're both pretty dark (but with some comic elements) and set in the late 19th/early 20th century. The plots resolve around a young boy with special powers and a strong determination. The two organizations from these shows (the exorcists and the alchemists) are somewhat similar as well.
Fullmetal Alchemist: Brotherhood and D. Gray-man share great artwork. The plot lines are similar as well, since they focus on an enemy and venture off to destroy said enemy. Lots of action and anime comedy (perversion jokes).
Allen Walker in D. Gray Man is in a very similar setting: orphan child who finds a family in a military environment.
The values are similar in these two series: friendship, loyalty, honor, fighting.
The adventures are more episodic in Allen's case. But the characters are very different: Ed is a very straightforward person: his mystery is elaborated in details right in the first episode, his motives are clear and unchanging, and he is kind of selfish. He is very impulsive and very loving.
Allen is on the other hand distant from other people, but he is lovable, too. He is different in one very important quality: self-sacrifice.
-Both feature a similar setting, with organizations of soldiers with special powers
-Both feature similar fighting methods, with some magic and some physical weapons
-Both have similar humor and a similar tone; as well as the same darker tone shift occurring during serious moments
-Main characters share some physical similarities and have personality similarities as well, though Allen grows out of his rude/brash phase
Both are fantasy anime set in more-or-less the same time period and feature protagonists with metal arms (Edward Elric and Allen Walker) who fight demonic beings created by artificial means (Homonculi and Akuma).
They also have many of the same voice actors (Roy and Kanda are voiced by Travis Willingham, Riza and Miranda are voiced by Colleen Clinkenbeard, Scar and Komui are voiced by J. Micheal Tatum, Ling Yao and Allen are voiced by Todd Haberkorn, Armstrong and Cross are voiced by Christopher Sabat, Kimblee and Krory are voiced by Eric Vale etc.)
Both shows also deal with similar themes, the most obvious being that the dead
Publicly mistaken characters that have lots of personality developments and have to go through great hardships that are very interesting to watch.
D.Gray Man focuses on the life of Allen Walker, who grew up with a deformed left arm, resulting in his family abandoning him claiming that he's not human. With his dark past behind him, a much darker future awaits him as the evil Millennium Earl plots to destroy the entire existence of humanity with his creation of Akuma. This weapon uses the souls of the dead which are then trapped within it until the use of Innocence, an Anti-Akuma weapon, breaks them free. After spending several years under the teachings of General Cross, Allen enlists in an organization called Black Order in hopes to prevent
The story is a masterpiece, at least for me. One of the most enjoyable anime out there. The MC has similar personalities. They have headquarters for monitoring the villains.