ギャル文字

Compound of ギャル (gyaru) +‎ 文字 (moji, “writing”), with the former element presumably borrowed from English girl or gal. The writing is associated with young people, especially young women.

ギャル文(も)字(じ) • (gyaru-moji) 

1. a form of cryptic Japanese writing that substitutes similar-looking characters for the characters intended, used especially in text messages
   • 2003 December 20, “Nippon ryūkōki: gyaru-moji”, in Yomiuri Shimbun, page 9:

     「（ナ”Ｗ（キ？」「八”Ｔ八”Ｔ」。それぞれ「元気？」「バイバイ」と書いてあるそうだ。「ギャル文字」とか「へた文字」と呼ばれるもの

     （ナ”Ｗ（キ？ 八”Ｔ八”Ｔ. Respectively, “Are you well?” and “Bye-bye” are written like that. It's something called “gyaru-moji” or “heta-moji”

2. the characters or symbols used for such substitutions

• へた文字(もじ) (heta-moji); 下手文字(へたもじ) (heta-moji)