Many fission products emit gamma radiation and our gamma-ray spectrometer measures gamma radiation directly. So Yes, a gamma-ray sensor can be used for this application, it is actually build to measure natural and man-made radionuclides directly. When we map sediments in the Netherlands, we usually map the fission product 137Cs which rained down after the Chernobyl accident.
We measure a complete spectrum of of gamma radiation and the intensity and shape of the spectrum is determined by the amount and energy of the radionuclides emitted. When we do not know which fission products will be present in the sediment we can assess the presence of fission products, when we know which fission products might be present in the sediment we can also assess the type and concentration of fission products
Assess the presence of fission products
We can determine the amount of man-made radiation and generate a "heat-map" that shows where fission products are present in the sediment.
An example of this kind of studies can be found in a paper on Measuring Radioactivity From Waste Drums on the Sea Floor.
Distribution of measured 137Cs radioactivity on the seafloor around waste drums.
Assess the type and concentration of fission products
Since we measure a complete spectrum of of gamma radiation, we can calculate the contribution of each fission product to this spectrum. This results in maps with the concentration of each fission product in the sediment.