During the High Luminosity phase, the CERN Large Hadron Collider (HL-LHC) will produce particle collisions at intensities about 10 times higher than its design value. In turn, this will induce much higher particle background rates in the accelerator and the effects integrated over a long period of time will present a new challenge for the LHC detectors such as the Compact Muon Solenoid (CMS). In particular, the high pseudo-rapidity region of its muon system is covered only by Cathode Strip Chambers (CSC) and lacks redundant coverage despite the fact that it is the most challenging region for muon detection in terms of backgrounds and momentum resolution. In order to maintain the present muon detection and triggering performance during the HL-LHC, additional new muon detectors will be installed in this region. In particular, additional Resistive Plate Chambers (RPCs) are planned to be installed in the endcap disks, in the region close to the beamline. Preliminary CMS RPC performance and stability results at the new GIF++ facility will be reported. The GIF++ is a new gamma irradiation facility to test detectors for the HL-LHC program; it is a unique place at CERN where high energy charged particle beams (muon beam with momentum up to 100 GeV/c) are combined with a 14 TBq 137 Cesium source. Plans for ageing studies at the GIF++ for the CMS RPC system will be also discussed.