...select the spot color in the swatches palette, choose the flyout menu, select "new tint swatch"...
...make it 50%, click OK...
...select the object that needs the gradient, get the gradient palette open and choose linear, drag your tint spot and the white paper color onto the gradient palette to get the gradient you need...
...you can also drag the created gradient to the swatches palette for use elsewhere and also edit the gradient by double clicking the gradient swatch in the swatches palette, this is a global change to any elements that use that gradient swatch...
andrew