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gain is just your 'iso' setting, not really increasing sensitivity but amplifying the signal you are already getting (noise and all)
i dont think it would be ideal to have it set as low as 0 but i can see why people would try that. from what ive seen with a quick play around a few nights ago, if your gain is low your offset will have to be higher to get within the 400 - 1000 range.
from what i remember my ETA Carina shot was taken with a gain of 20% and an offset of 116, i still have the bias frames and a quick check showed that my max pixel value for one of those was ~900.
the point i mentioned that is because my carina shot looks fairly clean, not a lot of noise but as soon as i had finished shooting the subs for that i also took a few of triffid (90 mins worth). the triffid frame was relatively empty though compared to the carina one and the background was pretty grainy after a hard stretch so im not sure there is an ideal setting for the camera to be on 100% of the time, if the frame is going to be full of nebulosity it might be better to boost the gain a little because you can get away with it but for smaller objects you will probably just get a noisy background
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