MXGS stands for Monitor X-ray and Gamma-ray Sensor and it is the high energy sensor in the Atmospheric-Space Interaction Monitor (ASIM). ASIM was proposed for the International Space Station external facilities on the Columbus module,  with the scientific goal of studying high-altitude optical emissions from the stratosphere and mesosphere, the so-called red sprites, blue jets, and elves  (or Transient Luminous Events, TLEs) and to provide evidence on the possible link between these optical emissions and Terrestial Gamma Ray Flashes (TGFs)
See ASIM page at the Danish Space Research Institute (DSRI).