I have an image created by a graphic designer and it has about 20 layers in it. How can I make an area transparent all the way through so that wen I export the image to PNG it will have a transparent "hole" in it?
You could apply an identical layer mask to each of the layers. Make one layer mask, copy it, paste it, and anchor it into another layer mask, 20 times.
You could hide the background layer (if there is one), and merge the visible layers to one layer, then add one layer mask - since it's being output to a png anyway, which has no support for layers.
You could hide the background layer (if there is one), then create one new layer from visible, and add one layer mask to it. Then hide all the other layers.
clear-layers deletes the selection in several layers.
Edit/Clear layers/Allaffects all layers, while
Edit/Clear layers/Visibleonly affects visible layers.
You can also do it directly in the python-fu console (warning, this assumes that there is only one image loaded in Gimp):
image=gimp.image_list() for layer in image.layers:pdb.gimp_edit_clear(layer)
(Strike enter twice after the second line.)