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.

On 21 January 2020, D.C. United acquired Gressel from Atlanta United for $750,000 in Targeted Allocation Money.
Gressel made his debut for D.C. on 29 February, 2020, in the season opener against the Colorado Rapids.
Gressel scored his first goal for D.C. United on 29 August 2020, in a 1–4 loss against the Philadelphia Union.
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.
On 21 January 2020, D.C. United acquired Gressel from Atlanta United for $750,000 in Targeted Allocation Money. Gressel made his debut for D.C. on 29 February, 2020, in the season opener against the Colorado Rapids. Gressel scored his first goal for D.C. United on 29 August 2020, in a 1–4 loss against the Philadelphia Union.
Julian Gressel