Interesting! I didn't imagine it would be so simple... I also didn't imagine getGammaOp() would be a public method in the viewer. I'm not sure it should be a public method, but it is, so this looks like a good fast solution for now.

Minimal example script to document use:

def path = buildPathInProject("rendered.png")
writeRenderedImage(getCurrentViewer(), path)

I confirm it applies the gamma to the image, while the overlay is unchanged.