Choose black as foreground colour and a colour you like as background colour.
3.
Now apply the the colours on the round mask using the gradient tool with the settings
shown in the screenshot on the right (for you english girls/guys: Opacity -80%,
Fore- to Backgroundcolour, Transparency&Dither checked).
Look at the screenshot where I applied the gradient: left to middle...
4.
Create a new Layer above the button.
Choose white as foregroundcolour and pick the airbrush-tool using the settings
shown in the screenshot on the right (for you english girls/guys: White light,
Pressure 48%, Fade 40 steps, transparent)
5.
Brush on the new layer 3-4 times as shown in the screenshot.
6.
Export to gif or whatever after reducing to background-layer.
Ready!
Try various settings for the gradient-fill and the airbrush
to find your own style...
