The term "
free software" was coined by Stallman. GPL was created by Stallman to promote free software. So yes, GPL'd stuff is indeed free. But it's not just free, it
also guarantees that anything built from it remains free. Free to use and change the code as you want, not free to make money off of it in any way you want (even if that seems to be the American Dream-definition of free)
There are also more copyleft licenses than GPL, for example CC-SA, which is sometimes used on code.
But for libraries, the common choice is
LGPL. Which allows people to release proprietary software that uses the library. But if they change the library they have to share those changes. Which makes a lot of sense.