First we want to open the project we want to apply this 2D effect to.
I have just quickly made this Orb for an example:

We are going to use Elliptical Marquee tool to make a selection.
Using shift so we get a perfectly round circle make a selection, we should make this quite a bit smaller than the circle we are using.
For example so this is my selection:

Make a new layer (Layer> New> Layer...).
Now using the gradient tool and using the color white going to transparent (), make a select into the selected area starting from the top going near to the bottom exactly like the example below:

We should now have something like this:

The edges are too sharp so we are going to use a gaussian blur (Filter> Blur> Guassian Blur...).
Use a radius of about 1.0 pixel.
Should now have something like this:

Only one thing left to do now, because the gradient stands out too much we are going to lower the opacity to about 60%.
WE'RE FINSIHED

We should have a result of something like this: