For the observation of solar or galactic gamma rays, one should ideally be above the atmosphere. Quoting the Wikipedia article on Gamma-ray astronomy:
Most gamma rays coming from space are absorbed by the Earth's atmosphere, so gamma-ray astronomy could not develop until it was possible to get detectors above all or most of the atmosphere using balloons and spacecraft.
If the atmosphere absorbs gamma rays and solar or galactic gamma rays are best observed from space, how can terrestrial gamma rays be observed from above the Earth's atmosphere?
In other words, what does the atmospheric optical depth look like at very high frequencies? Is the following diagram inaccurate?
Atmospheric opacity, from Wikimedia Commons, originally from NASA.