The Gain VOP is based on Perlin’s section in Texture & Modeling, and is currently only used in some of the older materials.

Where base is the Input, and amount is the Gain, the basic process is to take:

result = bias(2*base, amount)*.5;

when the base is greater than 0.5. If the base is less than 0.5, the computation is:

result = 1 - bias(2*(1-base), amount)*.5;
See also

VOP nodes