Gyaru-moji is sort of like a Japanese Leet: a variant orthography for Japanese. Unlike Leet, which was developed in the predominantly male hacker culture, Gyaru-moju (which means “girl characters”) appears to have been developed by schoolgirls. In both cases, by excluding those not “in the know”, use of the variant orthography serves to strengthen intimacy among the in-group.
Unlike the relatively small number (around one hundred) of printable symbols that computers usually allow in Latin text, Japanese textual communication has tens of thousands of glyphs to draw upon. For a long time, Japanese users have been able to type Katakana, Hiragana, Kanji (“Chinese characters”), Latin, Greek, Cyrillic, and a number of symbols — even on cell phones.
It is difficult to describe rules for Gyaru-moji; it seems that one of the rules is “there are no rules”. I will make an attempt, but understand that reality is more complicated than what I describe here.
Glyphs in one writing system are sometimes substituted for similar-looking glyphs in a different writing system, e.g. Katakana チ becoming Kanji 干, or Hiragana す becoming the section symbol
§. Characters with two disconnected sub-elements sometimes turn into two distinct characters, e.g. 私 becoming
禾+ム. (Note that Japanese writing is almost always monospaced — each character is centered in a square with the same amount of horizontal and vertical room. 私 takes one square;
禾ム takes two squares.)
Like Leet, Gyaru-moji uses unusual grammatical structures, but unlike Leet, those structures are not usually incorrect (by the standards of the dominant dialect), merely uncommon. Like Leet, there is not a one-to-one relation between a standard character and one Gyaru-moji character: there are many possible variations of each character.
Unlike Leet, Gyaru-moji does not appear to use the rebus principle (as Leet uses in “gr8” and “&hill”). However, they do sometimes transcribe characters phonetically in Latin, then alter the Latin characters. Thus も, the character representing the syllable “mo”, can become “мσ”. (I do not know if they ever would use the Leetish “AA()”.)