Probably "gals" went out of style in the feminist era of the 70s, when women objected to being called girls. It felt belittling. Especially since you'd see the term girl paired with man ALL the time.
"Boy" is seen as derogatory to men. Especially since white people used to call Black men "boy" all the time. So, when boy was dropped to refer to an adult male, girl (or the slang for that, gal) was also dropped.
Just like the "N" word being used amongst some AAs as a reclaiming, girl can be used amongst women. It is one thing to be belittled and patronized by being called a girl by men, it is another thing to feel enjoyably youthful when women bond with their female friends and call them girls.
Try this on: my girlfriend and I sleep and have sex together in the "master" bedroom. Should we call it the "mistress" bedroom?
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37