The darkest of the two will be the foreground and the lightest the background. Click on the flood fill tool and choose linear gradient on the tools control tab. After choosing a linear gradient, at the bottom of that tab is a button labeled options. Click on that and set the gradient to about 270 or so. Using the selection tool set to a circle, position your cursor on the center of your transparent canvas and draw a circle that is a little larger than the largest white section of your first canvas.
Flood fill the selection.
Choosing the darkest tone of the color you are using for the foreground, and the shapes tool set to circle and outline size 2, create the dark circle around the outside edge of the eyeball. Using the shapes tool set to circle and filled an choose black as your foreground color. Create the black section of the eyeball. Now, using white as the foreground color, and a new layer, place a highlight on the black section of the eyeball. Change to the smudge tool and very lightly smudge a little section to the outside of the eyeball as shown below.