Step 1: Create a selection around your original bottle cap image, either using the Ellipse select tool, or Fuzzy select tool (magic wand). If your bottle cap image is on a transparent background, like in the attached micki.xcf file, just right click the bottle cap layer and select "alpha to selection" to get your selection.
Save the circle selection to path. Select - To Path.
Step 2. Create a new white background above the bottle cap layer. Activate the circle path selection. Fill that selection with a medium gray color
(I used #7e7e7e). Turn off the selection (Select - None), apply a Gaussian blur at 30px. Reactivate the circle path selection, invert it (Select - Invert). Press delete to remove the outside excess. Turn off the selection again (Select - None). Click the eye on the blurred circle layer to turn it off.
Step 3: Select the bottle cap layer, go to Filters - Map - Bump Map and use the following settings. (Your target bump image is the grey blurred circle layer):

Step 4: On the bottle cap layer, go to Filters - Light and Shadows - Drop Shadow and apply the following settings:

That's it.
See attached file...
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"In order to attain the impossible, one must attempt the absurd."
~ Miguel de Cervantes