  After the Phase-2 high-luminosity upgrade to the Large Hadron Collider (LHC),
the collision rate and therefore the background rate will significantly
increase, particularly in the high $\eta$ region. To improve both the tracking
and triggering of muons, the Compact Muon Solenoid (CMS) Collaboration plans to
install triple-layer Gas Electron Multiplier (GEM) detectors in the CMS muon
endcaps. Demonstrator GEM detectors were installed in CMS during 2017 to gain
operational experience and perform a preliminary investigation of detector
performance. We present the results of triple-GEM detector performance studies
performed in situ during normal CMS and LHC operations in 2018. The
distribution of cluster size and the efficiency to reconstruct high $p_T$ muons
in proton--proton collisions are presented as well as the measurement of the
environmental background rate to produce hits in the GEM detector.
