Impact eliminated from playoff race with tie with Atlanta
Gressel scored in the 53rd minute, and Bojan Krkic tied it for Montreal in the 81st.
Defending champion Atlanta (17-12-4) has already clinched a playoff spot.
Clement Diop made five saves in his MLS debut for Montreal (11-17-5). Evan Bush was on the bench for the first time since the end of the 2017 season, a streak of 66 consecutive league matches.
Brad Guzan made three saves for Atlanta.