Can't remember if I submitted these (also not found on search here, and beta database unavailable):
As in attached and starting with gradient layer, black to white along x-axis. Add Luminosity Range Mask and adjust nodes as shown.
When curve hits top or bottom edge of adjustment square, the effect is sudden, unexpected and presumably incorrect.
Also when blur is increase, an edge effect around the periphery of the image is seen, much as would be found using a Gaussian Blur. It looks like a 'Preserve Alpha' checkbox is needed.