Gangster costumes are a popular choice for Halloween. In the peak gangster years of the 1920s and 1930s, women in gangs were usually known as "gun molls," the companions of male gangsters. They often dressed in alluring, fashionable clothes of the era, such as fringed flapper dresses and long strands of pearls. However, modern gangster costumes for women and girls are typically feminized versions of the suits male gangsters wore. The specific details of the outfit will depend on whether it is for a young girl or for a woman who prefers a sexier outfit.
Find a pin-striped suit, preferably black with white stripes or white with black stripes. It can be a pant suit or a skirt suit, and you may wish to find a coordinating vest for it.
Wear a simple white button-up shirt with a plain black tie, or a black shirt with a white tie.
Put on a pair of black, or black and white patent leather shoes. These can be simple lace-up shoes for young girls, or high heels for women who prefer a sexier look.
Pull hair back, and put on a fake mustache if you are trying to look like a male gangster, or play up your femininity by keeping your hair long and wearing bright red lipstick.
Wear a fedora. It can be black, white or pin-striped like the suit.
Carry a plastic toy gun to complete the outfit. You may also wish to have a fake cigar prop.