This plugin allows you to create layer mask from another layer mask. Or create layer mask from grayscale copy of layer.
This plugin requires Python!
If you have the "<Image>/Filters/Python-Fu/Console" menu item you are all set. If not, install Python support for GIMP.
If you have any questions, comments, or suggestions, please leave them
on this deviantart plugin page,
and I'll try to answer them.