Steve:
Draw a solid object, fill it with the color you want the gradient to start with.
Ctrl-C (Copy), Ctrl-F (Paste in Front), then apply a Black/White gradient to that copy.
Select the copy and the original, then on the Transparency Palette (Shift-Ctrl-F10), click the flyout arrow at the top right, and choose "Make Opacity Mask" from the menu. That should do it.
Bert