GCRs are mostly atomic nuclei from which all of the surrounding electrons have been stripped during their high-speed passage through the Galaxy.
Cosmic rays provide one of our few direct samples of matter from outside the solar system.
The magnetic fields of the Galaxy, the solar system, and the Earth have scrambled the flight paths of these particles so much that we can no longer point back to their source in the Galaxy. All of the natural elements in the periodic table are present in cosmic rays, in roughly the same proportion as they occur in the solar system. But detailed differences provide a fingerprint of the cosmic ray's source. Measuring the quantity of each different element is relatively easy, since the different charge of each element's nuclei give very different signatures. To tell the isotopes apart involves in effect weighing each atomic nucleus that enters the detector.