Gal's is the possessive form of gal, an informal word for girl or woman.
It indicates ownership or association: That's my gal's car, a gal's night out.
The word gal itself is casual and somewhat dated, more common in certain regional dialects and older speech.
1. informal — woman's, Female possessive form; lady's (Feminine possessive form); female's (Belonging to a woman); dame's (Belonging to a lady); chick's · sister's · girlfriend's · daughter's
2. possession — belongings, Things you possess; stuff (Unspecified items collectively); things (Objects in general); items (Individual units or objects); property · accessories · wardrobe · jewelry · handbag · shoes · makeup · clothes · outfit · phone
3. measurement — volume, Amount of space filled; capacity (Maximum amount containable); liquid (Free-flowing substance); container (Thing for storing items); measurement · unit's · quantity · fluid