Atlanta United FC
Gressel began his footballing career by playing in the youth academy systems of 2. Bundesliga side, SpVgg Greuther Fürth in 2002. In 2009, Gressel left Greuther Fürth and began playing for the academy set-up of Quelle Fürth. In 2010, he moved to Florida and attended the prestigious Pine School in Hobe Sound, FL where he also began playing for national showcase youth club FC Florida Prep Academy.
Before the 2013 NCAA Division I men's soccer season, Gressel signed a National Letter of Intent to play for Providence College's men's soccer program. During the 2013 season, Gressel was an immediate starter, starting and playing in all 22 fixtures the Friars played in. Gressel scored on his collegiate debut on 30 August 2013, in a 3–2 victory over Quinnipiac. In his freshman season, the Friars managed to reach the championship match of the 2013 Big East Men's Soccer Tournament, before losing 3–2 against Marquette. The Friars ultimately earned an at-large bid into the 2013 NCAA Division I Men's Soccer Championship, where they reached the second round before losing to the fifth-seeded, Maryland. Gressel finished his freshman year with five goals and four assists. He was also awarded with a spot on the 2013 All-Big East Conference Second Team, and on the All-Rookie Team.
During his sophomore and junior seasons, Gressel earned honors on the Big East Conference All-Tournament teams for both the 2014 and 2015 editions of the tournaments. Additionally, during his junior season, Gressell earned spots on the NSCAA Third Team All-Great Lakes Region and the Second Team All-Big East Conference.
Gressel had a breakout season during his senior year at Providence. He led the Friars in total points with 36, accumulating 15 goals and six assists his senior year. Gressel was also rewarded with the Golden Boot award for scoring the most goals during the 2016 NCAA Division I Men's Soccer Championship, during a quarterfinal run which saw Providence, beat CAA Champions, Delaware, upset top-seeded and Big Ten Champions, Maryland, and defeat Big East-rivals, Creighton. In addition, Gressel was listed an NSCAA First Team All-American and a semifinalist for the Hermann Trophy. Gressel additionally earned All-Great Lakes First Team honors, All-Big East Conference First Team honors, part of the 2016 Big East Men's Soccer Tournament's first team, and the All-Tournament team for the Rhode Island Capital City Classic.
Prior to Providence College, Gressel played senior-level football in the German regional fifth-tier, the Bayernliga. During the 2011–12 Bayernliga, Gressel had one goal and ten assists. The following season, Gressel played for TSV Neustadt/Aisch in the sixth tier, Landesliga Bayern-Nordwest, where Gressel notched three goals and 12 assists during the 2012–13 Landesliga Bayern-Nordwest campaign.
Ahead of the 2017 MLS SuperDraft, Gressel signed a senior college contract with Major League Soccer. Per league policy the terms were not disclosed. On January 13, 2017, Gressel was selected by Atlanta United FC in the first round with the eighth overall pick. Gressel made his professional debut for Atlanta United on March 5, 2017 in a 1–2 loss against New York Red Bulls.
On 8 November 2017, Gressel won the MLS Rookie of the Year honors.