[epiphanitus]

What could anyone hope to see pointing a gamma ray telescope at earth?

I thought gamma rays were filtered out though the atmosphere.

[TeMPOraL]

Detecting nuclear detonations? Or perhaps even tracking nuclear material?

AFAIR, it was thanks to such gamma ray detectors pointed down that we've accidentally discovered gamma ray bursts.

https://en.wikipedia.org/wiki/Vela_(satellite)#Role_of_Vela_...

[gumby]

Nuclear weapons testing I have always assumed.

[xxpor]

I assume not just testing, but the existence of a stockpile of fissile material in general, right? Obviously it'd need to be more sensitive but id have to assume if you observe a moving gamma ray source on earth that's really valuable information