The pole-sitting Pole led away at the start, though both Bamber and Nicki Thiim (Lechner Racing) kept him honest during the early stages.
But Giermaziak was able to keep the pair at bay and gradually build an advantage, pulling a second clear by lap 10.
From there he was unassailable and calmly brought the car home for his first win since the series’ trip to Red Bull Ring last month.
Reigning champion Thiim finished second with Bamber third. Spain’s Alex Riberas (Attempto Racing) took his best finish of 2014 in fourth, with Michael Ammermüller (Lechner) fifth and Klaus Bachler (Konrad Motorsport) sixth.
Giermaziak now leads Bamber by nine points atop the standings, with third-placed Ammermüller a distant 32 points shy of the lead. The series has four races to run over three weekends, the next at Spa-Francorchamps on August 24. It then travels to Monza before closing the season with a double-header event at Circuit of the Americas in November.