Second-seeded Atlanta United, the 2018 MLS Cup defending champions, will play host to fourth-seeded Toronto FC on Wednesday. The game will feature the reigning MLS Cup champion, Atlanta, and the 2018 champion, Toronto.
The third-seeded Union (17-12-7) saw their season end four days after the first playoff win in franchise history.
"We were secure with our passing, but also our opponent gets more tired and they have to run more behind the ball," Atlanta coach Frank de Boer said after the match.
"Also, what we showed today in playing four in the back is that we can adjust very quickly to another system, and that's difficult for an opponent. It was a very mature game."
Gressel scored after a series of passes with Gonzalo "Pity" Martinez delivering the ball that Gressel put away in the 10th minute of the match. It was Gressel's first career postseason goal in his eighth playoff game.
Brad Guzan made three saves in keeping the Union scoreless. Guzan robbed Philadelphia's Brenden Aaronson of a 17th minute goal with an athletic stop.
Atlanta had two chances within seconds of each other to add to its lead in the 60th minute, but Union goalkeeper Andre Blake, who finished with three saves, stopped both direct shots on goal.
In the 80th minute, Josef Martinez, who led Atlanta United with 27 regular-season goals, ran down a long ball from Gressel. He then used his speed to put enough distance between himself and Union defenders to get off a shot that soared past Blake and into the top corner of the net.
Josef Martinez has seven goals in seven career games against Philadelphia. He was close to scoring just before halftime but missed wide right on a header off a cross from Gressel.
Atlanta United had won its playoff opener 2-0 over the seventh-seeded New England Revolution on Saturday. Philadelphia edged the sixth-seeded New York Red Bulls 4-3 after extra time on Sunday.
Martinez seals win sends Atlanta to Eastern Conference final
Josef Martínez finds the net late to seal Atlanta's 2-0 win over Philadelphia at home. To watch MLS sign up for ESPN+.