Adjusting the color gray

Hello
And welcome to my world.
I have a Sony HS 20.
Using a gray scale and going into the user adjustment for custom 2 I find RGB Gain and RGB Bias.
What is the difference ?
Is the gain the power of each color ? and the Bias for adjust gray ?
I found that bias effects the adjustment just a little where as gain a lot.
Also looks like bias affects gray and the gain color. is that correct?
I got a good even correct gray using the bias one and leaving the gain alone.
also I found that custom 1 2 and 3 if adjusted the same will still look a bit different from one another.
is that the way it supposed to work ?
I used to have a bit of green and now just a very, very small amount of green.
So It looks like I did it correctly no ?

Also what is gamma correction and what does it do ?

Thank you !