gap moe ギャップ萌え

In anime, gap moe, or gyappu moe ギャップ萌え, is a type of moe where a single character or scene features two vastly different and perhaps opposing characteristics, unlikely to occur with each other. Specifically, the term refers to moe 萌え derived from this "gap" between the characteristics.

A classic example is a heartless school delinquent who finds a stray cat in the rain and decides to keep it. The gap between his usual uncaring self and his pet-caring self is considered gap moe.

Definition

Gap moe refers to a gap between what you'd expect a character or situation to be, based on its present characteristics, and a completely unexpected characteristic, that doesn't fit your expectation, or even common sense, at all.

Given this definition, many things that are gap moe are actually created so deliberately. The author starts with a stereotype, and adds a completely unfitting element that makes absolutely no sense to that stereotype.

Since we're talking about anime, the elements added are way worse that you'd imagine.

Something that's innocent, cute, or clumsy is taken, and mixed into something that's serious, violent, sexy, or erotic. Or the opposite happens. In various situations.

Note: an important point of this type of moe is to subvert expectations. Consequently, some instances of gap moe don't make any sense for people that aren't aware of character stereotypes and tropes commonly found in anime.

Examples

Appearance

Physique

Some examples of gaps in a character's physical attributes:

A pair of "large breasts," kyonyuu 巨乳, on characters that shouldn't have them. Like tomboy characters, "shrine maidens," miko 巫女, nuns, and so on.

On a loli, it's known as "oppai loli" in English, or rori-kyonyuu ロリ巨乳 in Japanese.

Hajimete no Gal
はじめてのギャル
(in the disturbing very first episode.)
Quack Experimental Anime Excel Saga
(only in the very disturbing final episode.)

Similarly, a young boy with a thick bears, betrayed his age.

Outbreak Company
(a random 10 year old dwarf background character.)

A little girl, but she's actually over 18. Known as "legal loli," gouhou-rori 合法ロリ.

Hunter x Hunter
(spoilers.)

Similarly, a little boy who's actually over 18.

Hellsing
(spoilers.)

The cutest, most feminine girl in the series, but she's actually a guy.

Steins;Gate
(Urushibara Ruka 漆原るか)
Yahari Ore no Seishun Rabukome wa Machigatte-iru
やはり俺の青春ラブコメはまちがっている
(Totsuka Saika 戸塚彩加)

A woman, but she's packing a lot of muscles.

Keijo!!!!!!!!
競女!!!!!!!!
(many characters.)
Koutetsujou no Kabaneri
甲鉄城のカバネリ
(Yukina 侑那)
Shinchou Yuusha: Kono Yussha ga Ore Tueee kuse ni Shinchou Sugiru
慎重勇者～この勇者が俺ＴＵＥＥＥくせに慎重すぎる～
Jormungand
(Sofia Valmer)

A girl with tremendous brute strength. Or a huge guy lacking it. This is the case with virtually any "female knight," onna-kishi 女騎士 in anime, as well as swordswomen.

Dumbbell Nan Kilo Moteru?
ダンベル何キロ持てる？
(spoilers.)

A little girl who can eat way more than her body weight in food.

Kemono Michi
けものみち
(Destroyer Hanako)

Clothing

Some examples in gaps in characters wearing clothing, costumes, and outfits that don't suit them include:

A character who's always wearing an uniform wearing casual clothes for once.

The act of crossdressing, specially if a guy still looks like a guy while wearing them, or a girl still looks like a girl while wearing them.

Wearing clothing for younger ages. For example, adult women wearing a sailor-style high school girl outfit, bloomers, or school swimsuits.

More radically, characters who aren't babies wearing baby outfits, pacifiers, diapers, and so on.

Character: Satanichia Kurumizawa McDowell 胡桃沢=サタニキア=マクドウェル
Anime: Gabriel DropOut (Episode 9, Stitch)

Personality

Some examples of gap moe due to gaps in the personality of characters, contrasting with their identity or appearance, include:

A character who's supposed to be cold liking girly, fluffy things, like teddy bears.

Angel Beats
(Shiina 椎菜)

A tall dude likes shoujo manga.

Gekkan Shoujo Nozaki-kun
月刊少女野崎くん
(main character.)

A character who dresses in punk fashion, or has a post-apocalyptic-looking mohawk, stereotypically featured by scary-looking characters, ending up being the nicest guy who takes care of elders, and so on.

An innocent-looking little girl, boy, or whatever, being actually a cold-blooded murder machine.

Men in Black (1997)
(a girl with a quantum physics book.)
Black Lagoon
(Hansel and Gretel)
Hataraku Saibou
はたらく細胞
(Macrophage.)
Youjo Senki
幼女戦記
(main character.)

A character who is quiet and doesn't talk much, except they talk more than anyone on the phone using text messages.

Steins;Gate
(Kiryuu Moeka 桐生萌郁)
Sayonara, Zetsubou-Sensei
さよなら絶望先生
(Otonashi Meru 音無芽留)

A character who appears nice, but is actually nasty. Including characters that don't talk much, but have very nasty thoughts inside their heads.

Himouto! Umaru-chan
干物妹！うまるちゃん
(main character.)
Ore wo Suki nano wa Omae dake ka yo
俺を好きなのはお前だけかよ
(main character.)
Watashi ga Motenai no wa Dou Kangaetemo Omaera ga Warui!
私がモテないのはどう考えてもお前らが悪い！
(main character.)

A character who is mistaken to be amazing by characters around them, but in reality is overwhelmed by other characters' absurd expectations of them.

OVERLORD
(main character.)
Handa-kun
はんだくん
(main character.)
Mairimashita! Iruma-kun
魔入りました！入間くん
(main character.)

A character who's supposed to be serious being actually clumsy. A character who's supposed to be extremely powerful dong extremely mundane things.

Hataraku Maou-sama!
働く魔王様！
(where a demon lord of another world works as a fast food employee.)
Quack Experimental Anime Excel Saga.
(Il Palazzo, head of the ACROSS organization, whose ultimate goal is world domination, starts with small steps first: city domination.)

Characters who are supposed to be evil, like bandits, demons, and so on, acting cutely, or clumsy, or doing good deeds, while characters are supposed to be good, like the police, the heroes, saints, the church, nuns, priests, angels, being actually evil, and scheming.

Gabriel DropOut
(where Satania is constantly bullied by Raphiel.)
Machikado Mazoku
まちカドまぞく
(where the frail Shadow Mistress Yuuko fights against a magical girl who has saved the world once.)

A character who's supposed to be a child acting like an adult.

A character who's supposed to be an adult acting like a child.

Mahou Tsukai no Yome
魔法使いの嫁
(main character.)

A child character who's actually a genius with knowledge far surpassing that of adults.

Nichijou
日常
(Hakase 博士.)

A character who has good grades academically, and is great at sports, but has absolute no common sense.

Kaguya-sama wa Kokurasetai: Tensai-tachi no Ren'ai Zunousen
かぐや様は告らせたい～天才たちの恋愛頭脳戦～
(main character.)

A character who has lived most of his life outside of Japan, knowing more about Japan than the average Japanese person.

Doushirou Degozaru
道士朗でござる
(main character.)

A character who's blonde, has blue eyes, but isn't a foreigner, was actually born in Japan, and happens to suck at English.

A character that's normally uncaring and rejecting, but sometimes shows their loving side to their romantic interest. A tsundere ツンデレ.

rooru kyabetsu-kei danshi
ロールキャベツ系男子
"Cabbage-roll type boy." (literally.)
A boy who's gentle and introverted, and appears to be soushoku-kei 草食系, "herbivore-type," but actually turns out to be nikushoku-kei 肉食系, "carnivore-type." The term comes meat rolled up inside a cabbage leaf.

Similarly, a character that's usually lame-looking and docile, but changes completely when something hurts their nakama 仲間 and they get angry.

The Incredible Hulk.
Mob Psycho 100
(main character.)
Kimetsu no Yaiba
鬼滅の刃
(spoilers.)

A virgin girl who's also slut. Yes, you actually read that right. Don't ask me, I don't make the words, I just list them. The term for this is shojo bicchi 処女ビッチ, a portmanteau of shojo 処女, "virgin (girl)," and the katakanization of the English word "bitch," which in Japanese means "slut."

Boku no Kanojo ga Majime-Sugiru Shobicchi na Ken
僕の彼女がマジメ過ぎるしょびっちな件
(main character's girlfriend.)

Stereotypically, gyaru ギャル characters, who dress gaudily, are assumed to be "easy," in the sense of having sex with anyone.

Since many anime have introverted protagonists, gyaru, and other riajuu リア充 characters, are often portrayed as nasty, sleazy, dumb, and as delinquents, smoking in school, among other things.

Consequently, gyaru characters that still dress gaudily, but otherwise have adorable personalities, study diligently, are actually smart, or perhaps even like manga and anime, are otaku, ends up counting as gap moe.

Relationships

Some examples in gaps between relationships:

The girl protects the guy, probably because she's stronger than the guy.

Attack on Titan
Mahou Shoujo Ore
魔法少女俺
Mirai Nikki
未来日記

In seme-uke 攻め受け dichotomy, where stereotypically the seme is masculine and assertive, while the uke is feminine and passive, any situation where an element differs from the stereotype. See also: types of seme and uke.

gyaku-nanpa
逆ナンパ
Reverse nanpa. Where a girl hits on and flirts with a guy. (the normal nanpa is a guy taking the lead and hitting on a girl.)
josou-seme
女装攻め
Crossdressing seme. Where the seme, and presumably top of the gay relationship, is a crossdresser. Normally, a guy who dresses up as a girl is seen as feminine and ends up being the uke and bottom instead.
gekokujou
下剋上
Where a subordinate is the seme, and their superior, boss, manager, master, teacher, or whatever, is the uke. Normally, characters in positions of authority are seen as dominant over their subordinates, who are seen as submissive, and the superior becomes the seme.
shotaone
ショタおね
A ship of a boy and an older girl, where the boy takes the lead. Normally, in oneshota おねショタ ships, the older girl is the seme instead.