Grzegorz Fijalek and Mariusz Prudel, the No. 7 team in the Olympic rankings, beat the Americans 21-17, 21-18. The U.S. team is 1-1 in pool play with one remaining match on Wednesday; they might not need to win, but a victory could be necessary to avoid relegation to a "lucky loser" playoff.
The Americans had beaten the Polish pair in straight sets at the Berlin Grand Slam event in mid-July.
"They made some big adjustments from the last time we played them," Gibb said.
Fijalek said it was obvious they needed to make changes if they were going to compete with the Americans, who were No. 4 in the world coming into the games.
"Of course, because we lost very easy in Berlin," he said. "We watched that game many times. We played new beach volleyball."
In the men's competition, straight set victories went to Martin Spinnangr and Tarjei Viken Skarlund of Norway over Canadians Joshua Binstock and Martin Reader; Aleksandrs Samoilovs and Ruslans Sorokins of Latvia over Freedom Chiya and Grant Goldschmidt of South Africa and Ricardo Santos and Pedro Cunha of Brazil over Britain's Steve Grotowski and John Garcia-Thompson. Germany's Julius Brink and Jonas Reckermann beat Wu Penggen and Xu Linyin of China 13-21, 21-19, 15-8.