Girl is a Noun, not an acronym, so it does not have any full form. In young years, a female child is said to be a girl. The word is specially used to talk about a female who is not married and is quite young.
Girl does not have any full form, but there could be some funny full forms of it like the one written below:-
G – Gussa hamesha naak pe
I – Innocent sirf shakal se
R – Rone ki automatic machine
L – Ladai mein sabki naani