Gain amplifies or attenuates the ambient light. You can use this to adjust your system to the environmental light. In most cases, 50% gain works well.
Shutter time is the amount of time that the camera sensor is exposed to light. In most cases, it works well when you have selected a medium shutter time such as 10 ms. If you set a low value, for example, 2 ms, the cameras will not detect all light sources within the exposure time. If you set a high value, the cameras will get too many interfering light sources and the measurement will fail.
Auto gain sets the gain automatically for the selected shutter time. This can help with the amount of light captured by the camera and make the scan easier in some cases.