The background is made from a plain purple color with added HSV noise. Then an effect is added on top of everything by copying the entire image into a new layer and applying a convolution with a kernel like this:

-1  0  0  0  0
 0 -1  0  0  0
 0  0  1  0  0
 0  0  0  1  0
 0  0  0  0  1

In GIMP those are under "filters > noise > HSV noise" and "filters > generic > convolution"