Layer mask gradual fade problem

[GMP]

I'm doing a face swap and the base layer skin was too dark, so I lightened up the base layer and it worked well for matching the skin on the face's neck and the upper torso of the base layer, But it doesn't look good for the lower part of the base pic's body.

So I then made a duplicate of the base layer and put a layer make on it. I can paint on the layer mask to reveal the lower layer near the neck. It now looks good near the neck, but I can't get the layer mask to have a very gradual fade to the belly button area.

Any suggestions?