The CMS Collaboration has been developing a Gas Electron Multiplier (GEM) detector in the endcap regions of the CMS muon system to maintain the high level of performance achieved during Run 2 in the challenging environment of the high-luminosity phase of the LHC (HL-LHC). The GEM detectors at endcap station 1 (GE1/1) were installed during the second long shutdown. The technical and operational challenges of large-area GEM detectors have been identified during the commissioning of five GEM super chambers (“slice test”) in Run 2. This led to a modification in its system design. A test with cosmic-ray muons is the final stage of quality control before the full-scale installation into the CMS detector. We review the performance of muon detection in the slice test, an improvement of the readout system, commissioning status, and prospects for the muon trigger for Run 3.