Doesn't Stack-anything fill that niche already, along with a multitude of youtube videos and online tutorials that will walk you through the most common use cases, especially for software like photoshop?

In case that's an actual question, make a rectangular selection and check the "rounded corners" box at the top left. Select the gradient tool (You can see it by clicking the little paintbucked on the left toolbar for a while, it looks like a gradient). Klick somewhere and drag with the tool selected. And there's your rounded rectange with a shaded gradient inside.