Is girl masculine gender of spinster?

No, in English there are no masculine or feminine forms. English uses gender specific nouns for male or female.

The noun 'girl' is a gender specific noun for a young, female, person.

The corresponding gender specific noun for a young, male person is 'boy'.

The noun 'spinster' is a gender specific noun for a womanwho is past a child bearing age but has never married.

The term stems from an era when most females were not educated and if not married became a dependent in the household of a family member. One of the jobs frequently assigned to an adult, dependent family member (everyone had to earn their keep) was spinning wool and thread, literally the family spinster.