MyFCOM VOL1 states that the gain adjusts receiver sensitivity to enhance ground mapping in MAP mode only.
In FCOM Vol 2 it states that gain should always be left in automatic,unless specific display enhancement is deemed necessary.
The majority of my colleagues use manual gain outside the terminal area as the radar appears to understate the weather.
I am currently looking for the make and model number of my aircraft's radar so that I can try an find a copy of the manufacturers handbook for it but I have never received any radar specific training.