For gray mappings gamma correction can be turned on by `set palette gamma <gamma>`. <gamma> defaults to 1.5 which is quite suitable for most terminals.
For color mappings no automatic gamma correction is done by gnuplot. However, you may easily implement gamma correction. Here is an example for a gray scale image by use of explicit functions for the red, green and blue component with slightly different values of gamma
Example:
set palette model RGB
set palette functions gray**0.64, gray**0.67, gray**0.70
To use gamma correction with interpolated gradients specify intermediate gray values with appropriate colors. Instead of
set palette defined ( 0 0 0 0, 1 1 1 1 )
use e.g.
set palette defined ( 0 0 0 0, 0.5 .73 .73 .73, 1 1 1 1 )
or even more intermediate points until the linear interpolation fits the "gamma corrected" interpolation well enough.