Bucs first-base coach Rick Sofield expressed his disagreement with the call on his way back to the dugout, contending that the pitch to Mercer had been low.
Sofield was tossed by Gibson, who also ejected Hurdle when the manager came to his coach's support.
The Pirates had one prior ejection this season, with Travis Snider having been tossed for his participation in a May 20 on-field fracas with the Milwaukee Brewers.
The Blue Jays led, 4-1, when Mercer took that inning-ending strike, which came with two men on. That was the first time the Pirates had managed to get two runners on base against McGowan.
Tom Singer is a reporter for MLB.com and writes an MLBlog Change for a Nickel. He can also be found on Twitter @Tom_Singer. This story was not subject to the approval of Major League Baseball or its clubs.