Using Plain color to fill a rectangle

To start, click the icon in the Fillsettings panel which does not show a gradient. The tooltip shows 'Plain old color'



Select the desired color. For example, pick an orange color



Using the fill rectangle tool, go ahead and dlick-and-drag the marquee rectangle to your desired size.



As soon as you release the mouse, it will fill it with plain color.



Using a Gradient to fill the rectangle

Now switch to the next fill mode in the Fill settings panel: horizontal gradient.



Draw another filled rectangle. This one will use the current gradient to fill the area.




If you need that gradient to go from top to the bottom instead of left to right, use this mode:  Vertical Gradient:



If you don't want it from black to white but instead from white to black, just reverse the gradient:



After reversing it, the gradient might look like this:



Here are two filled gradient rectangles with original and reversed gradient:



Here's a smaller one inside a larger one, after reversing the gradient: