2009-10: Gustav Possler skated for the Bjorkloven club's U18 and U16 teams in northeast Sweden and played for Vasterbotten in the TV-Pucken tournament for high school age players. He scored 9 goals with 16 assists and 24 penalty minutes in 25 games with the U18 team. In six games for Bjorkloven U16 he scored 9 goals with 4 assists. Possler had 6 goals with 3 assists and 14 penalty minutes in eight TV-Pucken games.
2010-11: Possler spit the season between MODO and Lulea in Swedish junior hockey and played in three U17 games for Sweden in international competition. He had no points in four games with Lulea's U20 team before moving to MODO. He scored 1 goal with 1 assist in one regular season game for MODO and was scoreless in three U20 playoff games. In 18 games with Lulea's U18 team he scored 21 goals with 15 assists and had 16 penalty minutes. Possler played in 16 U18 games for MODO, including three playoff games, and scored 8 goals with 9 assists and 6 penalty minutes. He had no points and 2 penalty minutes in three games for Sweden's U17 team.
2011-12: Possler made his pro debut as a 17-year-old, appearing in two games for MODO in Sweden's Elitserien, and represented Sweden in the 2011 U19 World Junior A Challenge and 2012 U18 World Junior Championship tournaments. He scored 1 goal and was +1 with the MODO men's team. Possler was the third-leading scorer for MODO's U20 team. He scored a team-leading 23 goals with 17 assists in 37 games and was +17 with 14 penalty minutes. In eight playoff games he scored 2 goals with 1 assists and was -1 with 4 penalty minutes. He skated in 11 games for MODO's U18 team, including one playoff game, and scored 9 goals with 4 assists. Playing for Sweden, Possler had no points nor penalty minutes in four games at the WJAC tournament and scored 4 goals with 4 assists and was +5 for Sweden's silver medal-winning squad at the U18 WJC.
2012-13: Possler played seven games with MODO's men's team and three games with Mora in Allsvenskan (the Swedish second league) and was the second-leading scorer for MODO's silver medal-winning U20 team. He scored 1 goal and was +1 with 2 penalty minutes in limited time for MODO. He was -1 with no points nor penalty minutes in his time with Mora. Possler scored 23 goals with 17 assists in 37 games for the MODO U20 team and was +17 with 14 penalty minutes. MODO finished fifth in the regular season but rolled to the playoff championship, defeating HV71 4-3 in the final. Possler scored 4 goals with 4 assists and was +4 with 4 penalty minutes in seven playoff games. He was ranked 64th amongst European prospects in Central Scouting's final rankings and was selected by Buffalo in the fifth round (130th overall) of the 2013 NHL Draft.
Possler is a scoring winger with good skating and stickhandling ability. He prefers to shoot rather than pass, and combined with his explosive first step, Possler can be a very opportunistic goal scorer. He has good hockey sense and uses his speed to create turnovers.
Possler will spend a few more seasons in Sweden before coming to North America. He is expected to spend the entire season with MODO’s senior team. . If he fully realizes his talent, Possler can be an effective scoring winger in the NHL.