[quote user="yiannis550"]I what is a high gain screen and if there are any disadvantages over a regular screen?[/quote]
Basically a screen designed to reflect more of the light being given out by the projector. The gain is expressed as a number relative to the light output available with a standard white board (the white usually being defined as magnesium oxide). So the white board is 1, anyhting with greater gain (or rather less loss!) is given as a number greater than 1, and screens with lower reflectivity have fractions of 1. So a common home cinema screen might have a gain figure of 0.9 or 0.8.
In general, high gain screens are more usually employed in business enviroments, where you might want to project with some ambient lighting on. Their disadvantages are usually a much more narrow viewing angle before the light levels drop off, and in the case of some high-gain screens this can even be seen as hot-spotting, where the image is brighter in the centre than it is at the edges.
That's why purist home cinema screens tend to be of lower gain, giving more even reflectivitiy and thus a wider viewing angle. Indeed, some screens can appear light grey rather than purw white, many home cinema enthusiasts feeling these give a more natural picture. But they do need to be used in near-total blackout.