gunner view mode

What you are referring to is an actual real life thing known as sight parallax (the optic the gunner uses to target is not inside the cannon barrel) which reflects the offset nature of how optics are mounted (left, right of barrel, or even roof like the Abrams) Your sight when set to a distance puts the round on target AT the distance you set. If for example the gunners optic is mounted to the right of the barrel and your optic is set for a further distance then what you are shooting at, the round will land to the right of where you aim, or left if the target is further then where you have the sight distance set.
The ability to toggle this on or off is only possible in RB and AB. For sim its always on (as it is in real life).

Cheers.