Gagne's wife, Valerie, gave birth to the couple's fourth child, a son, on Friday morning.
The Brewers reliever left the team earlier this week to be with his family and made it back just in time for the delivery, manager Ned Yost said.
Gagne's flight to Pittsburgh was expected to arrive at 6 p.m. ET. The quick arrival to the ballpark will not deter Yost from using Gagne in Friday's game.
"[It's] no problem," Yost said.
Second baseman Ray Durham, who left Tuesday's game with a right shoulder strain, is expected to be ready to play this weekend. The second baseman was listed on the team's batting practice rotation.
Yost, however, did not know for sure if Durham was fully healthy when the manager met with the media before Friday's game.
It was unlikely that Durham would receive much playing time in the first two games this series anyway, because of the two left-handed starting pitchers the Pirates have going in Games 1 and 2.
The switch-hitting Durham has faced left-handers only 73 times this season for a .219 average, while fellow second baseman Rickie Weeks has batted 113 times against southpaws at a .248 clip.
Weeks started at second base for Milwaukee on Friday night, batting leadoff.
Todd Krise is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.