Not biological son of King Gadhi. Father of Shakuntala.
Kaushika became sage Vishvamitra.
Read the story here
http://hindumythologyforgennext.blogspot.com/2012/04/sage-vishwamitra-part-1.html
http://hindumythologyforgennext.blogspot.com/2012/12/vishwamitra-and-parashurama.html