On the SGI, there is an equivalent to the Mac control panel mentioned above. The gamma utility. From the shell, run gamma with no arguments to see what the current system gamma is. The value that gamma returns is the value used to build the hardware Look Up Table (LUT). The default setting for the LUT on an SGI is 1/1.7, so "gamma" will usually return 1.70000. Sometimes you will hear this number referred to as the "system gamma" of an SGI. This is not correct according to the defintion I am using here ( My definitions).
The root user of the SGI system has permission to change the gamma in the LUT by giving the gamma program an argument. Typing "gamma 2.5" for example, rewrites the LUT with values determined by the curve 1/2.5.