The Hadron Blind Detector (HBD) is new upgrade detector for the PHENIX experiment at RHIC that consists of a windowless Cherenkov radiator directly coupled to a set of triple Gas Electron Multipliers (GEMs). The individual GEMs measure 22×27 cm2, and the top GEM in the stack is coated with a ∼300 nm layer of CsI that serves as a photocathode. The signal amplitude from the triple GEM stack is used to differentiate between single isolated electrons and overlapping electrons from close pairs. Therefore, the absolute gain of the GEM stack is a crucial parameter in understanding and interpreting the data. We accumulated extensive data on the GEMs during the design and construction of the detector, including gain variation with time, charging effects, saturation, gain uniformity, and source rate dependence. These results, as well as our experience in operating the detector during its first run at RHIC, will be presented at the Workshop.