If you would like to add a gradient over the layers, we don’t have an option for that. So in that case if you would create your own image file with a partially transparent gradient, you could try to use absolute positioning:
to put an image layer on top of your layers. But you should note, that layers weren’t made to cover the entire slide at any given responsive case, so there could be designs, where this couldn’t work out. If you have a test image, I suggest using that to check the responsive layout:
