It just does what it says on the deviantart page
This plugin allows you to create layer mask from another layer mask.
Or create layer mask from grayscale copy of layer.Unzip put create-layer-mask-from.py in C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\plug-ins
Do not bother with the locale folder unless you use Russian language.
Find the plug right-click in layer dialogue, near the bottom.
Saves a copy and paste, very similar to the older Saul Goode script but with more options.
From a black / white layer: example:
https://i.imgur.com/l0ISIP0.mp4From a layer with an existing layer mask: example:
https://i.imgur.com/yEbAIQ4.mp4