I used this question:
How to create a UIButton with a black gradient for iPhone?
... to set up a color gradient as the background of my button and it looks great, but when I tap the button it doesn't visibly change and give the user any feedback that they successfully tapped the button.
How can I set up a second gradient for the highlighted state or something like that?
Well, you are setting a sublayer and not providing any content to the button's views. The sublayers are drawn directly, so the UIButton doesn't think it has any content to "highlight". If you want a different effect on your button when you select it, there are three simple approaches.
One is to make two images with gradients. A pressed and normal state. This is much simpler as you can design them as you want and its a simple one line, one time implementation.
The other option is adding selectors that get called when your button gets UIControlEventTouchDown and whichever other Control Event(s) you want to have it switch back under.
The last simple option is to subclass UIButton and use the UIResponder touch methods to determine when to manually switch between the different background types.