This is one of those questions which I know you could garner significantly more information from the search function than I could ever give you. This is why I didn't answer it before.
I guess the short answer is that the gain number is the reflectivity on axis. So a 1.3 gain screen has 30% more light reflected to the prime viewing position than does a matte white 1.0 gain. The screen I use is 1.8 gain so it reflects 80% more light back to the prime viewing position than does a 1.0 gain screen and so on. This is a simplistic answer since some gray screen coatings like that on a FireHawk complicate this a little.
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