Gain is the reflection index of a screen.
It indicates the ratio between the light reflected by the screen and that reflected by a standard white surface used as a reference parameter:
a screen with 1.0 gain will reflect the same amount of light, while a screen with 1.5 gain will reflect 50% more light, and a screen with a 0.8 index will reflect 80% of the light.
The gain is measured in the point where the screen is at its brightest, that is watching it from a frontal and perpendicular position.
Moving to the side and watching the screen from an angled position, the brightness of the projection decreases.
''Is high screen gain good?
It is easy, and wrong, to jump to the conclusion that a high gain screen must be preferable to a low gain screen.
First of all, there is a compromise between gain and viewing angle.
In high-gain screens, the brightness of the projection decreases considerably with the increase of the viewing angle, while in low-gain screens the brightness varies much less perceptibly.
Furthermore, a high-gain screen does not generally reflect evenly red, green and blue and the anomaly varies with the viewing angle.
Lastly, screens with a gain higher than 1.0 have a certain degree of hotspotting which is accentuated with the increase of the gain.
Than means, that looking the screen from a frontal position, the central part of the image appears brighter than in the peripheral areas.
This is not very visible up to a gain of 1.3, but, beyond that limit, hotspotting can seriously disturb vision.
''[ ... ] the videophile looking for the optimum image quality
[ ... ] will usually want to opt for a low gain screen.
Source: Evan Powell, in ProjectorCentral.com