A gal pal is a female friend. Gal is an old, informal way to say girl or woman, and pal is a casual word for friend; they were put together because they rhyme. It is chatty and a bit jokey, and in real writing it is very often two words: gal pal. It can mean a close woman friend, though actually people also use it lightly, even in gossip, without meaning the friendship is deep. It is not a formal word, and to some ears it sounds dated — the kind of thing a magazine might say, not a school essay.