What is the meaning of "Zonah"?
Radak: She was a concubine. She is called a Zonah because she did not have Kesuvah and Kidushin, like a Zonah, even though she was designated for him.
Radak citing Targum Yonasan: Pundakisa. This is like he translated regarding Rachav (it can mean innkeeper or harlot; refer to Yehoshua 2:1:4:1,2 and the notes there).
Radak citing Targum of Tosefta, R. Bechayei (Bamidbar 36:8): The custom was that a man would not marry a woman from another Shevet, lest inheritance pass from Shevet to Shevet. When a man wanted to marry a girl from another Shevet, she would not get a dowry from her father's house, and people would call her Pundakisa who loves a man from another Shevet. So was Yiftach's mother. 1
Radak: The Targum said so, for his brothers told him "you will not inherit with us, for you are from another woman" (verse 2). According to my Perush (she was a concubine), they said unlike the Halachah. "You expelled me from my father's house" (verse 7), i.e. improperly.
Why does it say "va'Yoled Gil'ad Es Yiftach"?
Malbim: [Even though his mother was married to Gil'ad,] all knew that Gil'ad fathered him, for he designated her to be his concubine.
