  Large area triple GEM chambers will be employed in the first two stations of
the MuCh system of the CBM experiment at the upcoming Facility for Antiproton
and Ion Research FAIR in Darmstadt/Germany. The GEM detectors have been
designed to take data at an unprecedented interaction rate (up to 10 MHz) in
nucleus-nucleus collisions in CBM at FAIR. Real-size trapezoidal modules have
been installed in the mCBM experiment and tested in nucleus-nucleus collisions
at the SIS18 beamline of GSI as a part of the FAIR Phase-0 program. In this
report, we discuss the design, installation, commissioning, and response of
these GEM modules in detail. The response has been studied using the
free-streaming readout electronics designed for the CBM-MuCh and CBM-STS
detector system. In free-streaming data, the first attempt on an event building
based on the timestamps of hits has been carried out, resulting in the
observation of clear spatial correlations between the GEM modules in the mCBM
setup for the first time. Accordingly, a time resolution of $\sim$15\,ns have
been obtained for the GEM detectors.
