Add a border around an element, and apply a gradient to the border, starting from dark at the top to light at the bottom (Assuming a top-down light source)
If you want to create a "frame" effect around an element, like adding a frame around a photo, you can make it look like it has an inward chamfered edge by using a gradient. This is because in real life, light coming from the top would hit the bottom edge more, making it look lighter, and the top edge less, making it look darker.
If you're using a gradient on the element as well, like in the example below, it can be hard to achieve this effect because of how the colour in the element interacts with the colour in the outline. It might take some tweaking.