Diffraction grating works by taking photons striking it at any given wavelength, and spreads its energy out into a gradient across the spectrum. Gen 2 tubes have an extremely flat and even response curve shown below.
These tubes compared to GAas have lower overall sensitivity, with a wider flatter profile that is not conducive to the night sky.
Gallium arsenide photocathodes are perfectly tuned to maximize efficiency across the red and nir, which is the vast majority of outdoor light in the darkness, its the reason that Gen 3 tubes are so much better, their sensitivity and responsiveness are perfectly honed in for the night sky.
The reason this technology is not implemented in Gen 3 tubes is that adding a grating would work against this, as its splitting light across the spectrum away from where it needs it most so in effect it would just hinder performance.
