to the left : gradient color in the blending options of the layer
to the center : a separate layer to draw the gradient color
to the right : a simple black and white gradient applyed in a layer mask
And :

on the first row : 3 different gradient colors on the associated background colors
on the second row : I have added a white background to let you see the gradient (which have the same opacity)
on the third and last row : what is happened if we change the background color for the three methods

only the third method (to the right) is still adapted to the background color.

Please, do not forget to add a perspective effect on the layer « reflection » to make it look better (Edit+TransformTheLayer+Perspective) (for Photoshop but it should be exactly the same method for Gimp)

To help you, I have attached the sample PSD file to this tutorial to let you see the three methods in action
The PSD file of reflectionexample (111) - 146.41 KB