Overview
Gaki(ガキ) is used for both boys and girls.
It is a derogatory term that emphasizes the child's impudence, so care must be taken when using it.
Origin of the word
The origin of the word "Gaki" is from the Buddhist word "餓鬼".
It's easier to understand if you think of it as a hungry goblin.
They are punished by constant hunger, and even when they have food or drink, it turns into fire in their hands.
Mesugaki
In English-speaking countries, it may be better known as Mesugaki.
Mesugaki means female brat.