Gamma rays have the shortest wavelength and highest frequency of all EM radiation. Only the very highest energies can reach the surface, the rest are absorbed by Ozone in the Earth's upper atmosphere.

Gamma rays are produced in areas of extremely high temperature, density and magnetic fields. Gamma ray observations were first taken in the 1960s on the Apollo and Ranger missions.

The first sky surveys were done in the 1970s by the SAS-2 and COS-B followed up by the HEAO satellites in the late 70s and Granat in the early 90s. Gamma rays of above 100GeV require instruments larger than can be carried on satellites and for these energies the Earth's atmosphere itself is used as a detector, with optical telescopes used to record the Cerenkov radiation produced by the photons.


References:

Snow T.P. & Brownsberger K.R. (1997) Universe: Origins and Evolution
Ridpath I. (1997) Oxford Dictionary of Astronomy