"gyaru" (ギャル) is a Japanese word that is loaned from the English word "Gal". It's a fashion subculture that's stereotyped by blonde hair and spray on tans, as well as loud clothing (animal stripes, neon colors).
As a personality or character, gyaru are stereotypical delinquents. They tend to be around high school or college age as characters. (Though not always, I know of one great one where the female character is a gyaru MILF.) They tend to fill roles as bullies. They tend to be unapologetically brash and straightforward. They're also stereotyped as slutty, being the sort to leap on men just for the heck of it as they walk down the street. Given their age, they also tend to gravitate towards enjou kousai (compensated dating, sort of a form of prostitution. Or extortion, depending on the girl.) and other delinquent/slutty behaviors.
TL: DR-- Gals/gyaru are stereotypically slutty delinquents with a specific visual style.
