
The gradient tool in Photoshop can be useful in a digital illustration of any kind. It creates a realistic and smooth shading which can help in achieving a better quality result than painting with the brush tool. Combined with the lasso tool or a blending mode it creates countless possibilities to achieve creative results.
In The latest version of Photoshop CC, all you need to do is go to the gradient tool, from the top menu click on the arrow next to the gradient preview icon and expand the library. In the Basics folder you will find the Foreground to Transparent gradient.

Now, set the gradient opacity to around 33% and it’s ready to use!
Here is how you can manually transform your gradient tool into a great shading tool in Photoshop:
(Skip these steps if you already have a ready-made Foreground to Background gradient)
Here is a short video showing all of the steps below:
Step 1: Choose the gradient tool from the left menu and go to gradient editor.

Step 2: You should have a basic gradient with two opacity stops, two color stops and a midpoint stop in the middle. Click on the color stop icon to the left and the arrow beside the color swatch. Choose ‘Foreground’ from the drop down menu.

Step 3: Click on the left opacity stop icon and set the opacity to 100%.

Step 4: Now choose the opacity stop to the right and set the opacity to 0%.

Step 5: Name your gradient and exit the editor. In the top menu set the gradient opacity to 33%.

Done! Now your gradient will transition from the color you have currently selected to zero opacity. Time to try it out!
On a white background the radial gradient will look something like this.

I added some radial gradient to the monochrome drawing below.

Setting the gradient layer blending mode to Overlay can produce a nice effect when adding color to a monochrome illustration.

Here is a quick demo of how the gradient can be used as a shading tool. The gradient layer is set to multiply and I also used the lasso tool to select areas I wanted to shade.