That step will work only with one colored shapes.
I got another idea that might work:
I'll start with this, to show how you do with non rectangle shapes (and shapes with more than one color):
Duplicate layer, and go to Adjustments > Brightness/Contrast (Ctrl+Shift+C).
Turn the brightness all the way down and contrast all the way up.
You shold now have a black shape on the top layer (with the same shape)
Invert the layer (Ctrl+Shift+I).
Now, you'll have to select the part of the gloss you don't want.
(Draw a line on a new layer and select the area below)
On the layer that's mostly white by now, delete.
Then delete the layer you used for the selection.
The edge will be a bit jaggy, so select an area around the edge (just be careful, you don't want to have the upper corners in your selection), and blur or feather it (don't too much though, just make sure it's kind of smooth).
Then adjust the opacity.
Hope this will help you!