First Create a rounded rectangle. Corner Radius should be around 5 px.

Duplicate this shape. Change the bottom layer’s color the the color you want your button to have.

Apply these settings to the top layer:

As well as these:

Now to the bottom layer, apply these settings:

Now create a highlight with the rounded rectangle tool again. Apply the same settings as you did to the top layer.

Duplicate these 3 shapes.

Right click and select “Merge Layers”. Now you should have just one layer.

To flip it, press Control-T. Now change the height to -100%. Make sure “Constrain Proportions” is off.

Move it down and this is what you should have:

Apply these blending options to make it lighter:

And also lower the opacity to 30-50%, whatever looks best.

Now you can add whatever text you’d like and your glossy button is complete: