Apply a gradient to existing lines?

[Donny Bahama]

I have an intricate design that I created using the Spiroshapes plugin. I'd like to recolor the (black) lines using a gradient - but I can't do a color replace from one layer to another, can I? I tried putting the gradient in a different layer, then selecting the black lines, then inverting the selection, switching to the layer with the gradient, and deleting. But that looks like bloody potato because there's no anti-aliasing.

Is there any way to do this?

[Sarkut]

If you have black lines on white, make your gradient on a layer below the lines layer.

Set the lines layer Blending mode to Additive.

[yellowman]

Or use Alpha Mask, use the black lines as a mask on the colored layer.