The current state of the GEGL Dropshadow op is very restricting. It only works on transparent parts of the layer.

How to reproduce:
1. Create an image.
2. Select an arbitrary region of the current, non-transparent layer.
3. In Tools/GEGL operations... apply the Dropshadow item.

Expected result:
Like the current Non-GEGL Drop shadow filter: a new layer with the shadow is generated. If nothing is selected or the selections bounds are equal to the layers bounds, then a bigger shadow layer is created to fit the shadow.

Actual result:
If there are no transparent areas on the layer or the shadow would be outside the layers boundaries, then no shadow is generated.