It is difficult to separate the various wavelengths of incident gamma rays, given that they are very short and very close to one another.
In the case of light, this could be achieved with an optical assembly of prisms. In the case of gamma rays, monocrystal scattering is used, but since scattering angles are extremely close they are measured with a laser interferometer.
The actual device is very selective; it uses two perfect monocrystals and lets through only a very narrow wavelength band.