When a video signal is sent to a display device, the intensity of the picture should - ideally - match the value of the signal. So, if we send a signal at 50% of its maximum value then the display should be 50% of its maximum intensity. In real life, this does not happen. The intensity of a CRT display is not linear to the level of the signal sent to it but is described by a power function where the exponent is the gamma of the display. Video camera and video transfer operators add the inverse of the gamma function to a signal to compensate for this. In video, the gamma is 2.2 so output=input^2.2.
Digital projectors do not have a gamma response like CRT's, so they must emulate the same "gamma curve" in order to produce a correct picture. Most projectors have a gamma correction facility, which will usually been seen as an ability to have gamma presets, such as Computer, Video, Film etc.