The high-luminosity LHC (HL-LHC) upgrade is setting a new challenge for particle detection technologies. In the CMS muon system based on gas detectors,the increased luminosity will yield a ten times higher particle background compared to the present LHC conditions. To cope with the high-rate environment and to maintain the actual performance, new Gas Electron Multiplier (GEM)detector swill be installed in the innermost region of the forward CMS muon spectrometer, 2 <η < 2.8 (ME0 project). The detailed knowledge of the detector performance in the presence of such a high background is crucial for an optimized design and efficient operation at the HLLHC. A precise understanding of possible aging effects of detector materials and gases is of extreme importance. For this reason, aging tests of full sized triple-GEM detector operated with an Ar/CO2 (70/30) gas mixture at an effective gas gain of 2×10^4, are in course at GIF++, the CERN Gamma Irradiation Facility. One detector is irradiated with 662 keV gamma - rays from a 14 TBq 137Cs source and, in parallel, a second similar detector with 22 keV X-rays at the quality control lab. This contribution describes the performance of triple-GEM detectors during the irradiation test and reports on their state-of-the art.