Screen gain is the measure of the reflection of light from a video screen. No screen can actually create light, instead high-gain screens focus more of the light towards the viewer. Negative-gain screens absorb some of the light with the intention of creating better black levels.
Many factors go into the decision as to what gain is best for your specific front projector. For example, the amount of ambient light in the room or the amount of light emitted from your projector. Positive-gain screens offer more light than a "neutral gain" screen. Stewart's popular Studiotec 130, for example is a 1.3 gain screen. Their Grayhawk screen is an example of a negative gain screen.
It is advisable for even skilled home theater installers to consult with top screen manufacturing companies before placing that final order, as the right screen material can make or break a front-projection video system.