A gentlewoman is a woman of a high social class — the old female match for gentleman. In past centuries it could mean a woman of good birth, or a woman who served a queen or a great lady. You will meet it in history and in older stories. Almost nobody uses it in ordinary talk now; we say lady, or just name the person. Though actually, it does not mean 'a woman who is gentle and kind'. Gentle here is the old 'well-born' sense, the same as in gentleman. A kind shop worker is not a gentlewoman in this meaning, and a cold, proud woman of rank still was one. Some writers today bring it back on purpose, to sound old or to balance gentleman.