"Fifty-seven resigned in disgust because of the treatment of two of their members, who were simply executing orders," PBA president John Evans said.
The 57 officers have not resigned from the police department, only the ERT.
Gugino is in stable but serious condition in hospital.
The 75-year-old man was captured in shocking video being pushed to the ground by police officers in Buffalo amid protests over the death of George Floyd.
Gugino is a longtime peace activist who has protested for causes including nuclear disarmament and action on climate change, according to Buffalo News.
A friend described him as a "gentle person" who would never clash with authorities.
"He would never resist physically any kind of orders … he's a bit frail, not because of his age. He has some health problems," Terrence Bisson told the newspaper.
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The video taken by a reporter from the local public radio station WBFO, showed Gugino approaching a line of officers in riot gear.
One officer pushes him with a baton and a second one with his hand. The sound of a crack is heard and then blood trickles from his head.
"I was deeply disturbed by the video," Buffalo Mayor Byron Brown said in a statement. "After days of peaceful protests and several meetings between myself, police leadership and members of the community, tonight's event is disheartening."