Born in the province of Bergamo, he grew up in the youth teams of Bolgare and then Atalanta, who loaned him to Palazzolo and Montichiari, before returning to Bergamo. He made his Serie A debut for the club on 14 March 2010 in a league match against Parma, coming on as a 79th-minute substitute for Simone Tiribocchi. Following his debut, Gabbiadini made one further appearance for the club during the 2009–10 Serie A campaign.

On 9 July 2010, Gabbiadini joined Serie B side Cittadella, along with Daniele Gasparetto, in co-ownership deal for a peppercorn of €500, as part of the negotiation that saw Atalanta sign Matteo Ardemagni. He scored five goals in 27 appearances, including 15 starts, during the 2010–11 Serie B season.

On 17 November 2010, Gabbiadini made his debut with the Italian under-21 squad in a friendly game against Turkey. On 29 March 2011, he scored his first goal for the U21 team in a friendly game against Germany. On 6 October 2011, he scored a hat trick in the away match won 7–2 against Liechtenstein. He took part at the 2013 UEFA European Under-21 Championships in Israel, scoring two goals against hosts Israel in the group stage, as Italy went on to reach the final, only to lose to Spain.

On 21 June 2011, Gabbiadini returned to Atalanta after his co-ownership deal was resolved for €1.5 million for 50 percent of his registration rights. During the 2011–12 Serie A campaign, Gabbiadini was fully integrated into the club’s first team. He made 25 league appearances for the club, including eight starts, and scored his first Serie A goal on 25 March 2012 in the 2–0 home victory over Bologna. He also scored in his lone 2011–12 Coppa Italia appearance for La Dea.

On 24 August 2012, Juventus officially signed Gabbiadini in a new co-ownership agreement for €5.5 million, however it involved €3.5 million cash along with the co-ownership of James Troisi’s contract, which was valued at €2 million. Troisi was a free agent just two days prior to the deal, but signed by Juventus for the sole purpose of this negotiation.

On 15 August 2012, Gabbiadini made his debut with the Italian senior team, playing in the second half of the friendly match against England in Bern, which Italy lost 2–1.

Immediately following his transfer to the city of Turin, Gabbiadini was loaned to fellow Serie A side Bologna on a season-long loan deal. The loan deal was not confirmed until the final week of the 2012 summer transfer window, though Gabbiadini previously trained with Bologna days before the deal was officially finalized. He enjoyed a productive year at Bologna, scoring seven goals in 31 appearances during the 2012–13 Serie A season. On 19 June 2013, at the end of the campaign, the co-ownership agreement between Atalanta and Juventus was renewed, with the player’s registration rights remaining with Juventus.

On 9 July 2013, Sampdoria acquired the 50 percent registration rights of Gabbiadini from Atalanta for €5.5 million, with Juventus retained the remaining 50 percent registration rights. Gabbiadini would spend the 2013–14 Serie A for the Ligurian club. In his first season with the Blucerchiati, he scored ten goals, a personal best, in 35 appearances.

In January 2015, Gabbiadini joined Napoli for a reported fee of €13 million, to be split equally between Sampdoria and Juventus, each owning 50 percent of the player at the time of the deal. (Juventus later declared that their revenue was €6.25 million). He made his debut on 11 January against former team Juventus at the Stadio San Paolo, replacing José Callejón for the last 17 minutes of a 3–1 defeat. His first goal for the team came on 1 February, the winner in a 2–1 victory away to Chievo from Ivan Strinić’s assist. On 19 March 2016, he scored a first-half brace in a 6–0 victory over Bologna.

On 17 November 2015, he came off the bench to score his first international goal for Italy in the 66th minute of a 2–2 friendly home draw against Romania, in Bologna, although he was forced off early after sustaining an injury.

On 31 January 2017, Gabbiadini signed a four-and-a-half year contract with Southampton. The transfer fee paid was reported to be in the region of £14 million to £15 million depending on the source, although the club did not officially disclose the transfer fee. On 4 February 2017, he scored his first goal for the club on his debut in the Premier League in a 1–3 loss to West Ham.

He started his second game for Southampton on 11 February and scored twice inside the first 45 minutes in Southampton’s 4–0 win at Sunderland. He scored two more goals in his next match (and had a further goal disallowed incorrectly for offside), in the 2017 EFL Cup Final on 26 February, in a 3–2 defeat by Manchester United to bring his goal-scoring tally to five in his first three matches for Southampton. With his goal in the next match, a 4–3 win at Watford on 4 March, Gabbiadini became the first Saints player to score in each of his first four matches for the club. His performances for Southampton throughout the February period saw him awarded the Premier League’s PFA Fans’ Player of the Month award.

Gabbiadini’s older sister Melania is also a forward, who has spent most of her career with AGSM Verona and plays internationally for the Italy women’s national football team. He has mentioned her as a formative influence. On 17 June 2017, Gabbiadini married his long-term partner Martina Rubini in the parish church of Sasso Marconi, Bologna.

On 11 January 2019, Gabbiadini returned to Italy and signed with former club Sampdoria, for a reported fee of £12 million.