I learned this trick from Juan, of course
You can use
PixelMath to create a gradient image, and use them as a mask. I attached a screenshot and the PM process icon (right click, Process Icons > Load Process Icons to open it). In this example I create an greyscale image of 800x600 px, with a gradient from bottom right and 800 px radius. You can modify this variables in the Symbols field (cx, cy, r) to create you own mask.
I hope it helps.
Enzo.