Creating layer used ONLY to control final transparency

[todf3dfo]

I would like to create a layer which is only used for the final transparency.
The layer's RGB to be completely ignored and have no affect on the final merging of the layers.
This is used to "cutout" parts of the final image.
I already have a layer with a mask which is used for semi-transparency. I would prefer not to touch this layer.

Is it possible to set this up in Gimp?

The screenshot of my current setup.
http://imgur.com/a/aXtXM

The DIFF layer is used. The layermask is for semi-transparent areas.
I want the cutout layer to only make certain sections of the final image completely transparent.

Thanks for the help.

[Ofnuts]

I don't think you can do that with a layer. What you can do is save such as mask as a channel. To finalize your picture, Layer>New from visible, and add a layer mask, which you initialize with the saved channel.

[todf3dfo]

(07-19-2017, 09:16 PM)Ofnuts Wrote: I don't think you can do that with a layer. What you can do is save such as mask as a channel. To finalize your picture, Layer>New from visible, and add a layer mask, which you initialize with the saved channel.

Thanks for the help. This does the trick.
Never in a million years would I have figured this out by myself.

I decided to use the following workflow:
create a "cutout" layer,  fill with all white (0% transparency)
paint areas to be cutout as black (100% transparency)
create a "cutout" channel
copy contents from "cutout" layer and paste it into cutout channel
choose Layer->New from visible
then add layer mask to the new layer, and choose the "cutout" channel as the mask