The gradient is not a tool you use very oftne when it comes to your photo restoration projects. This tool is used far more often in areas like web site design and development and graphic design. The gradient tool basically allow you to draw out an area that has a gradient going from one colour to the other or a spectrum of different colour. This tool can be very using for creating good looking effects on websites and is used quite often to create backgounds with one colour at the top blending into another colour at the bottom.
The gradient tool can also be used on text and shapes to create a professional looking effect and such areas where this effect is needed. As i mentioned above, when it comes to photo restoration you will rarley if ever use this tool. You can also change the opacity of the colour on the gradient so you can blend when part of the graient into another layer if you so desired. You can also have the gradient despersed with a variety of effects. linear is the default effect but you can also have radial. which is basically the patterns developing outwards with circles.