I imagine you could select the different sliders for the lips and mouth and set them to negatives to get close to the desired effect.
I think you might even be able to use something like this:
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And just select the surface for the gag and change it to match the skin. Maybe?
But honestly any photo editing software or even an online photo editor would do something like that real easily. And with multiple ways to get the same result.
You could use a wand/clone tool to select part of the forehead and just copy it over the mouth. You could select an area for the heal tool to source from and apply it to the mouth. You could even use the color replace tool to draw over the mouth.