Gentile is a minor character and a player of Italy Jr. Youth. He supposedly plays as a forward and wears the number 12 jersey during the J Boys' Challenge story arc per Captain Tsubasa 3109 Nichi Zenkiroku. In reality, though, his original appearance in the manga is a mistake, in which he appears with jersey #9 (actually used by Golbate) and the play in which he originally appeared was later portrayed as being done by #4 Federico in the 2001 anime.
Gentile is a different character from the defender Salvatore Gentile or the eponymous forward from the game Captain Tsubasa J: Get in the Tomorrow, although the latter appears to be inspired in Gentile's position combined with Salvatore physical appearance with a different personality.
He is one of the forwards of Italy Jr. who faced Japan Jr. in the International Jr. Youth tournament.
(Source: Captain Tsubasa Wiki)