With just a linear gradient, mask still doesn't look like it has the right perspective. Delete the mask we tested with. Therefore, we will apply the masking in Vanishing Point using a separate pre-made gradient image. Create a new layer, then launch the Vanishing Point filter. The existing grids will show, and paste in the gradient image. Drag the gradient to the desired position. Apply the changes, and fill a new layer with black - this will merge with the gradient, and will become the mask.