I'm not sure how the GIMP file is set up, but it looks like you're approaching this the wrong way.
It seems like you've added a purple color on the layer with the card art on it. Is this correct? And to make it fade to white toward the middle of the card, you used a gradient layer mask.
What you should have done is simply create a new layer above the art layer, fill it with the purple you want, then create the gradient mask (without a selection; let it do it to the whole layer), and set the layer's Mode to Multiply or Overlay or Soft Light, or just play with others Modes and see what you like best.