Gmoch replaces Nioplias at Panathinaikos
Gmoch has taken over as caretaker coach until a long-term replacement is found.
Nioplias, 45, has been under pressure because of the team's defensive style of play. He threatened to quit last month but was persuaded to stay on.
"I leave the club as a friend and with the belief I always acted in their best interests," he said in a statement on Monday.
"We experienced some incredible moments that will remain in my memory including winning the double last year and I fervently wish the team repeat similar successes this year."
Nioplias replaced Henk ten Cate last December and ended a six-year trophy drought by leading Panathinaikos to the league and cup double.
Panathinaikos drew 1-1 at home to Larissa on Saturday. They have the same points as Olympiakos Piraeus but are top due to a superior head-to-head record.
However Nioplias's team are bottom of Champions League Group D, with only two points from four games.
Gmoch previously coached the club from 1983-85.
"I am a soldier at the disposal of Panathinaikos... it is not the time for statements but for hard work," said Gmoch.