Create a new file, 100x100 pixels.
Use the selection tool, make a square.
Choose a dark foreground color and a light backround color.
With the gradient tool set on Linier.
Click on the top-leaft corner and drag to bottom-right.

Click SELECT>MODIFY>CONTRACT, and enter "6".

ReSelect the gradient tool.
click bottom-right to top-left to draw gradient.

Once again click SELECT>MODIFY>CONTRACT, and enter 3.
Click on SELECT>FEATHER and enter 2.

With the gradient tool click top-left and drag to bottom-right.

There you have it. an easy to create button.