Alright so I don't know if this is the right way to do it but what I found looking through the script files and other forum posts is that this is related to the gun in question.
So if you want to decrease the "mouse sensitivity" for looking left and right so you can be more precise with your shots you will have to change a value in the gunspec file for the gun you want to use.
If you were to use the magnum you would go to
Gamecore/guns/modern/Magnum357
Then you would find gunspec.txt
In that you will find the following values
zoomwalkspeed = 0.7
zoomturnspeed = 0.7
plrmovespeedmod = 1.0
plremptyspeedmod = 1.0
plrturnspeedmod = 1.0
plrjumpspeedmod = 1.0
plrreloadspeedmod = 0.5
These are the coefficients I believe is what you call them that the engine will multiply to come up with the final speed with which you turn.
The one you are interested in is
plrturnspeedmod
As you can see it is set to 1.0
I play with a really low setting in all my FPS games like CoD and CSGO
So, I set this value to 0.2
Then you have to reload the map or just restart GG. Now your turning with the gun will be slower.
You will have to do this with any and all guns. Hope it helps someone and saves them some time.