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Gain is ISO with your DSLR. I use ISO1600 almost exclusively with my 1100D because it seems to be the sweet spot for the signal-to-noise ratio. Besides changing the gain, the only way you can increase the light in your image is by taking longer exposures.
The histograms represent the distribution of the pixel values across the whole image. What the RGB histogram is showing is that the majority of the pixels are low in value or close to zero (black). This should be the same as the histogram you can see if you press the Display button on the camera while reviewing the image you just took on the camera. What you want is to get the histogram shifted away from the left hand edge so there's clear flat line between the left hand edge and where the histogram ramps up.
If you're drifting visually for 10 minutes at that magnification, you shouldn't have any problem pushing the exposure with your DSLR.
I'm not sure about the diagonal line in DSS though, I've never seen that behaviour.
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