I add a new layer on top of it and near its mouth area I add a small circle and apply gaussian blur to it, and set the mode of the layer to overlay to create this(I used this merely for demonstration):
Now, suppose that I wanted the same effect more intense and that I duplicated the same circle layer two times, to create this: (all of them in overlay mode)
Now, How do I keep the effect of the three overlay layers but make them only one single layer? If I merge them down, the effect goes away:
How can I do this in GIMP?
EDIT: Just for the clarification for other folks whom I directed to this post, another reason to want to do this is when we create a layer mask for the above layer with overlay we may have to copy and paste the layer mask to every overlay layer and making edits to layer masks even troublesome.