The color determined by the lighting/shading model is computed and stored in the variable color. We think of this as computing the color in linear space. There is no consideration of the monitor's response during the calculation of the shading model (assuming that we don't access any texture data that might already be gamma-corrected).
To apply the correction, in the fragment shader, we raise the color of the pixel to the power of 1.0 / Gamma, and apply the result to the output variable FragColor. Of course, the inverse of Gamma could be computed outside the fragment shader to avoid the division operation.
We do not apply the gamma correction to the alpha component because it is typically not desired.
