Is a generalization, such as saying "All girls do not understand," considered backbiting (gheebah)? And is talking about an unknown person on the radio considered backbiting?

Gheebah (backbiting) is your mentioning about your Muslim brother something that he dislikes. Backbiting only occurs when specific, named individuals are mentioned. The statement, "All girls do not understand," is not backbiting because it is an incorrect generalization. Similarly, if someone says, "The people of such-and-such a town are stingy or bad people," that is not backbiting. However, speaking about a Muslim in a way that he dislikes is forbidden, whether on the radio or elsewhere, and whether you know him or not.