Short answer: No. Long answer: What do you mean "big doses"? There are many sources of gamma rays in the atmosphere (when stuff like cosmic rays hit it, you get a nice shower of gamma rays and other neat thingies). Maybe if you have a gamma spectrograph you can filter out just the cobalt-60 gamma rays, assuming they're unique? In that case you just need to worry about the fact that the surface is huge and gamma detectors are non-directional. That means that to scan a point on the surface you need to point straight at it. Unless you have a massive constillation of sats that means each "square" you scan will need to have a pretty high CPM for there to be a statistically significant number of counts during the scan. Due to the inverse square law, your satellite in LEO will only see a few CPS if somebody within 1KM of the source is getting several MILLION CPM. That translates into radiation sickness within a few days. For being 1KM away. Don't even ask about being in the same room as it! And of course the area you're scanning in 1 second is pretty huge so this detector wouldn't be much help locating things. And that's assuming no background radiation on the same order (or higher) CPS.
This would change if you have a gamma ray vector spectrograph that lets you measure the exact frequency and vector of each gamma ray it detects. But right now I think the filters are pretty fuzzy AND the techniques used are all non-directional. Even assuming perfect filters and vector detectors, the counts have to be huge before they show up in space right when you're looking. And I think the assumption you even can filter so you won't see any background ticks is incorrect, but I have no idea what kind of spectral distribution the Earth's gamma background has.
The reason you can have satellites that detect and locate the gamma bursts of underground nuclear tests is because of the B word. If it's a burst then you can triangulate between satellites even though their detectors are scalar not vector. That's because the sudden uptick that each satellite sees is tied to the same physical event. If you're looking at decay emissions then the counts are not synchronized so you can't triangulate. Oh, and also the gamma ray burst from an explosion is pretty big compared to the decay from a few kg of cobalt-60.