I don't know what the range in gain values is for the ASI120, but every camera has an acceptable range of gain. What you will find, however, is that individual imagers are all over the map with gain recommendations, and many will obtain excellent images with very different settings. This is largely dependent on the seeing you have, and how many frames you stack. If you have excellent seeing, you can take relatively long exposures with lower gain, slower frame rate, and sometimes get a superior result than using high gain and high frame rate. But in poor or average seeing the reverse is usually true. You need enough experience to make a judgement call based on the live view of what type of shutter speed you need. Bright objects like Mars can take incredibly fast shutter speeds (2.5ms and less) while dimmer objects like Saturn require much longer exposures. This also depends on your scope used.
I use Firecapture instead of Sharpcap, but the basic principle of interpreting a histogram is that the number that is listed (and constantly fluctuating somewhat) is telling you what the brightest pixels are registering as, with 255 being the brightest number on an 8 bit camera (pure white). If anything registers as pure white, the data is lost. This is called "clipping". Planets do not have a wide dynamic range, so you do not need to maximize the spread of the histogram like you generally want to in daytime photography. You also want to leave enough room on the right hand side of the histogram for sharpening, which moves it to the right (makes it brighter). If you captured at a 90%+ histogram you would almost certainly clip some regions to white after sharpening. But even more important is that if you capture at too high a histogram, it means you are either using too slow a shutter speed, which results in a low frame rate, or your gain is too high, which yields a noisy image. You can't go wrong with a 50% histogram on Jupiter, Saturn, or Mars. Some people go higher, and some go lower. But 50% is not a bad place to start.
As far as maximizing frame rate, this can make sense, but with modern cameras and small ROIs, you can encounter a situation in which the maximum potential frame rate of the camera is faster than you want. Take this for example. On Mars, with a small region of interest, I can attain over 400 frames per second with my ASI224mc. Because Mars is so bright, I can use a moderate gain value to achieve this result. However, with Jupiter, I could also theoretically attain the same frame rate by using a 2.5ms exposure, but doing do would require me to use a very high gain and the result is actually worse than if I use a much slower exposure of around 6-8ms and a lower gain value.
This brings up the following observation. Faster frame rates, if achieved by using high gain values, do lead to more frames captured. However, in order to eliminate the noise, you now have to stack more frames. A slower frame rate with a longer exposure and lower gain will lead to less frames captured, but you can achieve the same SNR with fewer frames stacked. So there is a range of parameters in which the results will be identical. But there is also a point at which the results diverge. For example, in good seeing, I use a 25ms exposure on Saturn, which limits me to 40 frames per second. However, the result is superior to what I get with a 10ms exposure, higher gain, and 100 frames per second, despite getting more frames that way. Results with 15ms and 20ms exposures are largely similar to the 25ms exposure, but use different gain values, to keep the histogram at 50% or slightly below. In average seeing I use the 15ms exposure, but in good seeing I consistently get better results with a 25ms exposure and lower gain. It's a good exercise to experiment with different parameters and see which gives you the best result under which conditions.
Also, you can adjust gamma to help you focus in live view....BUT return gamma to "50" before capture. Otherwise it distorts your data in undesirable ways. Gamma can (and should) easily be adjusted in post-processing, but not before.