The Hide Gradients option toggles the rendering of gradient fills (linear and radial) in the drawing editor. This option is located on the Fill Tab in the Info Old portion of the sidebar. Click on the box located next to the Fill label to hide or show gradient fills. Refer to the images below:
The following image examples show what a figure looks like with this option turned on and off.
If gradients are hidden, that does not mean they have been removed from the drawing. It simply means that they won't be drawn in the editor to increase rendering speed.