Pretty straight forward question, say in Gimp I have an image with 2 layers in question, one is temporary, I want to use it as the mask for the other. How can I transfer a layer into the mask of another? I get as far as having a grey scale floating selection which I can't place on the mask. I know how to paint on a mask, or use a layer's grey scale as it's own mask, but not a layer's grey scale for a different layer's mask.
Hope that makes sense
Assuming you are using white for opacity and black for transparency and, when making the layer mask, you chose the second option, "Black (full transparency)":
Right click on the floating layer and select "Anchor Layer". The image contained in the floating layer will replace the solid black mask.