The technique You need at least 3 layers. One for your drawing, one for the highlights and one for the shadows. Here, the drawing layer is the blue circle. A selection has been made on the highlights layer and floodfilled with a lighter color. Gaussian blur has been applied to the selection and the selection released.
Gaussian blur has a wide range of blurring.
The higher the setting, the more transparent the layer gets and the more it lets the underlaying layer show through. Using a darker shade of blue, the same process has been applied to the shadow layer . Flood fill 3. Larger selection 4. Gaussian blur 5.