[9] In the playoffs, Kitchener made it to the conference finals where they were eliminated by Windsor, and Gabriel was third overall on his team as well as the highest scoring rookie ahead of teammate Ryan Murphy.

During the playoffs, Landeskog posted a team-high 10 points in a seven-game first-round series defeat to the Plymouth Whalers to end his junior career.

[14] He made the Avalanche roster for the 2011–12 season; one factor in this decision was possibly that it enabled the team to reach the floor value for the salary cap.

[15] Landeskog scored his first NHL goal on 12 October 2011, against the Columbus Blue Jackets to send the game to an overtime win.

Landeskog was the first rookie to score his first NHL goal as an equalizer in the final minute of the third period since Tom Gilbert of the Edmonton Oilers did it in 2007.

[17] Landeskog finished his season with 52 points, including a team-leading 22 goals, surpassing Matt Duchene as the youngest in franchise history to do so.

[20][21] In 17 games with the mid-table Djurgårdens IF, Landeskog produced six goals for 14 points before returning to North America citing income tax issues on 3 December 2012.

On 27 July 2021, the day before becoming a free agent for the first time in his NHL career, Landeskog signed an eight-year, $56 million contract extension to stay with the Avalanche.

[34] After a strong beginning to the 2021–22 season, in early March 2022 it was announced that Landeskog would undergo knee surgery in order to deal with an injury that had been "nagging" him for some time.

[36] Landeskog was a strong performer through the first three rounds of the playoffs, helping the Avalanche advance to the Western Conference finals for the first time since 2002.

[41] A month later, the Avalanche reported that Landeskog's right knee required a cartilage transplant surgery, as a result of which he missed the entire 2023–24 season.

[8] Landeskog's rise was rewarded the following season as he was named an alternate captain for the 2011 World Junior Championships[44] but only played one game before he was sidelined with a high ankle sprain.

[47] In 10 games, Landeskog contributed with three goals and four points in helping Sweden claim the gold over Switzerland, becoming the first host team in 27 years to do so.

[48] In July 2013, Landeskog was one of 35 players invited to the Swedish Ice Hockey Association's orientation camp for the 2014 Winter Olympics in Sochi.

He was later included in the final squad and helped Sweden claim a silver medal in a 3–0 win over defending champions Canada.