A gal is an informal way to refer to a woman or girl.
When people talk about someone being a gal, they often mean it as a friendly term. For example, if a friend introduces herself and says "I'm Sarah, by the way," and another person responds with "Nice to meet you, gal!" They're not using it in a formal sense - they're just chatting and being casual.
How common is "gal"?
Word gal is considered rare in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
noun
A unit of volume, equivalent to eight pints
Exactly 4.54609 liters; an imperial gallon
231 cubic inches or approximately 3.785 liters for liquids (a "U.S. liquid gallon")
One-eighth of a U.S. bushel or approximately 4.405 liters for dry goods (a "U.S. dry gallon").
(in the plural) A large quantity (of any liquid).
Example: The pipe burst and gallons of water flooded into the kitchen.