The half gain angle is the area of the room for which the projected image produces the fullest spectrum view. This area is roughly in the shape of a cone with its point toward the center of the screen. Viewing cones are described by defining the degree radius formed by the outer limits of the cone. Higher gain reflective screens produce narrower viewing cones, while matte surfaces produce a broader viewing cone. Half gain angle is only 1/2 of the viewing cone. Multiply the half gain angle by two to get the total viewing cone.

'ganho medio' means 'medium gain', which is not quite the same as half.
The idea is half of the peak in the graph of the possible gain.
The expression is actually half gain viewing angle, so angulo de meio do ganho maximo de visao should probably be our choice. It reflects half of the maximum that is possible, therefore comes half the way in terms of reflectivity.