Duplicate the layer and desaturate (image> adjustments > hue/saturation) it until the red becomes really dark grey and the detail of the gradients shadow is still visible. Then goto image > adjustments > levels and you should see to big spikes. The left one is the darker colour that was the red. Drag the sliders below the be around the right spike and you should only be able to see the white. The left slider should be just to the left of the 2nd spike, the right slider should be just to the right and the middle slider centered on the spike. Click Done
Now select all of the image, and copy to the clipboard. Select the original layer and create a layer mask. On the channels panel select and make visible the one named mask and paste into it. Click back to the layers, and click back to the image.
It should now be transparent. You may need to play around with it a bit but that is the general process I would use.