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I was doing some reading up recently and one thing that stood out was that the lowest worthwhile gain is unity, whatever that works out to be for your particular camera. Essentially unity gain means one captured photon should translate to one step ADU, less than that and you essentially have a non linear response to actual captured light. After that, exposure times will come in to it as a higher gain would more quickly bring signal up above the noise, but if you are using longer exposures then brighter stars will saturate pixels more quickly as well so it is a trade off and depends on what you are trying to capture.
If you are using a gain for instance where it takes 1.5 captured and converted photons to increase the ADU count my one, one photon captured will be an ADU count of zero, two will be ADU1, three will be ADU2, four will still be ADU2, five will be ADU3, six will be ADU4 etc.
Basically to capture the data most effectively you want at least one step in ADU per photon captured and converted.
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