I really don't know in which subforum to post this, so I put it here.
Just so you know, GIMP's overlay mode is completely broken.
It is identical to "Soft Light". They're not fixing this because everyone got used to it.
Here is how to emulate Photoshop's Overlay mode in GIMP:
Use "Hard Light" but with the layers reversed.
This is important in the context of creating normal maps and a number of other things.
GIMP bug: https://bugzilla.gnome.org/show_bug.cgi?id=162395
Here I used 3 layers, with the GIMP normal map plugin. Corrected overlay (inverse hard light) clearly produces the smoothest result. Quite a difference from the first version, huh.