The Blue Jays pulled even in dramatic fashion on Saturday, when J.P. Arencibia's pinch-hit RBI single with the bases loaded in the bottom of the ninth capped a rally and lifted them over Baltimore, 5-4.
Gregg then uncorked a wild pitch to plate Johnson, and later in the at-bat Arencibia laced a single to score pinch-runner Chris Woodward and end the game.
The hit made a winner of Jesse Litsch (6-3), who allowed one run on three hits in two relief innings.
Jose Bautista launched a two-run homer and Lawrie added a solo shot for the Jays, who have won four of six and shook off a 2-0 loss in Friday's series opener.
Henderson Alvarez got the start for Toronto, allowing nine hits and three runs over seven innings.
Chris Davis collected two hits and drove in a pair of runs for the Orioles, whose three-game win streak was halted.
McGowan, a 29-year-old from Savannah, Ga., was a supplemental draft choice of the Blue Jays in 2000 and made the majors in 2005, when he went 1-3 over seven starts and 13 appearances.
He returned for 16 games in 2006 and had his longest stint in 2007, when he was 12-10 with a 4.08 earned run average across 27 starts and pitched 169 2/3 innings.
Only 19 starts followed in 2008, however, and McGowan did not return to the big leagues until a four-inning relief appearance against Boston on Sept. 6, in which he allowed three runs on five hits in a 14-0 Toronto loss.
He is 0-3 in eight career meetings with the Orioles, with a 6.53 ERA.
A supplemental draft pick in 2007, Hunter was 9-8 in 22 starts in 2008-09 before breaking out to win 13 games in 23 appearances last season to help Texas with the American League championship.
He was winless in eight starts to begin this season, however, and came to Baltimore along with infielder Chris Davis.
Hunter is 2-2 with the Orioles and has faced the Blue Jays twice since arriving. He gave up eight hits and four runs on a 5-4 loss at Baltimore on Aug. 5, then got a no-decision after giving up six runs on nine hits in six innings of an 8-6 loss on Sept. 1.
Toronto has won 11 of its 17 matchups with the O's after going 15-3 in the season series a year ago against them.