Sorry, I mean at least 300-500 yards away.

If you use black, you can add reflective markers or signs so people will see it before trying to drive through it at night. White will always look dusty.

it depends on what else is near the gate? trees, shrubs, a hedge or will it stand alone?

It won’t immediately be near trees. Stands alone in the sense that it covers the width of the road (dirt mountain road) and has a drainage ditch on one side + drops into a field on the other.

Shiny, smooth finishes are easier to wipe clean of bird droppings. Have a can of matching spray paint for touch ups.

People do black, because most of the "metal" fencing is also black. The black recedes. The white STANDS out and not in a good way!