A matt white screen ill reflect the image equally, whereas a high gain will be more directional and have a viewing 'cone'. If you move from say left to right across the screen, you'll notice it'll be slightly brighter in the middle than at the sides.
Do you need a high gain screen?
To know this, you'll need to tell us the screen size and projector.
A high gain screen will look brighter than a unity gain screen, all things being equal. It will also make your blacks less black, and that may be an issue for you (it is for some, but not everyone).