This begs the question : what exactly is it that you want to quantify ?

But from my window, the quick answers would be :

• on the GL histogram of your values : do the usual statistics, easy in ImageJ using the histogram & measurements option
• consider the GL profile as you would a roughness profile and you can then compute all sorts of "GL roughness" parameters (lots of information around on the web on Roughness Parameters)

To my knowledge, there exist only a few plugins for roughness work on ImageJ, such as :

Filtering : https://imagej.net/ij/plugins/waveness-roughness.html

Surface : https://www.gcsca.net/IJ/SurfCharJ.html

My inclination is generally to export the profile and do the computation outside ImageJ (Excel is sufficient for the most common parameters but I use MatLab or Mountains for topography computation)