From 2007 to 2009, Gentile played with the junior teams of Treviso Basket. In 2009, he signed to play with Treviso's senior men's team and in the 2009–10 season, he played in 24 games in Italy's top-tier levelLBA league, averaging 3.4 points and 1.2 rebounds per game. In 2011, he won the Italian League's Best Player Under 22 award. On June 14, 2011, he signed a three-year contract extension with Treviso Basket.
In December 2011, Gentile left Treviso Basket and signed a four-year deal with Olimpia Milano. In 2013–14, Gentile had a break-out season, averaging 11.4 points, 2.4 rebounds and 2.3 assists over 21 games in the EuroLeague. In June 2014, after Milano won their 26th Italian League championship, Gentile was named the Italian League Finals MVP. In July 2014, Gentile penned a new three-year contract with Milano. A year later in 2015, the Milano captain extended his contract until 2018, with a new three-year deal. On October 6, 2015, Gentile played against the NBA's Boston Celtics, in the NBA Global Games tournament. Gentile was sent on loan to the Greek club Panathinaikos in December 2016. However, Milano retained his rights through the 2017–18 season. He was released by Panathinaikos in March 2017, and he finished the 2016–17 season by playing on loan for Hapoel Jerusalem in the Israeli Super League, and preparing for the upcoming EuroBasket 2017 tournament.
Panathinaikos (2016–2017)
On December 5, 2016, Gentile reached an agreement with his Italian team, and he left Milano to conclude his season on loan in a non-Italian team. On December 19, 2016, Gentile then joined the Greek club Panathinaikos, on a loan from Olimpia Milano, for the rest of the season. He was with Panathinaikos, as they won the 2017 Greek Cup Final against Aris. He was then released by Panathinaikos in March 2017. With Panathinaikos, he averaged 3.2 points, 2.2 rebounds, 0.6 assists, and 0.3 steals per game with them in the EuroLeague, and 5.8 points, 2.5 rebounds, 1.6 assists, and 0.6 steals per game in the Greek Basket League.
Hapoel Jerusalem (2017)
On April 3, 2017, Gentile signed with the Israeli club Hapoel Jerusalem, of the Israeli Super League for the remainder of the Israeli Super League 2016–17 season. However, he was released by the club before their season ended. In 8 games with the club, he averaged 4.3 points, 2.0 rebounds, and 0.8 assists per game.
On June 26, 2014, Gentile was selected with the 53rd overall pick in the 2014 NBA draft by the Minnesota Timberwolves. His NBA draft rights were later traded to the Houston Rockets on draft night. On August 19, 2018，Gentile was reported to have intention to join the Houston Rockets, but no news was announced since then. Later Gentile was waived by Houston Rockets on October 12.
Gentile is the son of the Italian basketball icon Nando Gentile. He is also the younger brother of Italian professional basketball player Stefano Gentile. Alessandro moved around Italy and Europe, following his father during his playing career. Along with his older brother Stefano, he played with the youth teams of Panathinaikos in Greece, while his father played with the club's senior team. He has played with his brother in the senior Italian national team and Virtus Bologna.
