Gimp: Copy layer 1 to layer 2's mask

Question

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

Thanks

Answer 1

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.