I’ve tested at home, with GIMP 2.9.5 running on Ubuntu 16.04, and it works (output blending mode ‘overlay’ really sets it to overlay).
Thanks I’ll rty again at home, maybe I did something wrong. which I didn’t notice yet.
Now I get it!
As the duscussion was about output into Gimp layers I thought pos was the position in the layer stack. So it’s no surprise that all my tries with 1, -1, 2, -2 etc… didn’t affect the layer position and weren’t really visible, because I couldn’t see the pixel shift.
Ah OK. The order of the layers is just given by the order of the image in the G’MIC stack, the top layer being
 and the bottom layer being
[-1]. This info thus don’t need to appear in the image name.