Here is the comparison with and without vertical gain correction.(The step on the right hand edge is outside the corrected area)
It flattens the profile as expected but note the artifact where the dust on the slit was, caused by the dispersion direction not being exactly aligned with the rows (tilt). This is an extreme example of course but there must always be some small errors across the whole flat if there is any tilt. (Note when measuring these column profiles accurately any slant/smile should be removed otherwise this can distort the profile measured in a vertical slice)
The profiles were produced using a rather obscure French program called Teleauto which I found when I started doing some photometry. (I forget the name of the author now)
http://www.teleauto.org/indexEn.php
It is obsolete now (only handles 16 bit images and struggles to run properly on win7) but it has some cool unusual features, particularly for its day (~15 years ago)
Cheers
Robin