This doesn't fully apply to my original question but it helped me solve the situation that the question arised from. Also, I might add pictures when I have time to make it easier to understand.
What I originally wanted to do was to take an image and make it "fade" out into transparency. That is easy enough to do when working with the gradient tool and layer masks. You simply draw a black and white color gradient inside the layer mask.
The problem here, however, was that, using the linear gradient tool, I could not control the way the gradient would "progress", it would always end up cutting things off in a straight line, something that I thought made the resulting image look a bit wierd.
So I wanted to "curve" the gradient, to make it look nice and smooth.
However, this was not possible with the gradient tool. Which leads me to my solution.
Reading Rob's answer I realized that I could probably achieve the desired effect by simply using the lasso to make my "path" (the way I want the gradient to be shaped), then refine the edge to give it a nice feather and ultimately delete that selection from the layer mask. I realized that this would do pretty much exactly what I wanted.
So that's about it. It turned out to be alot easier than I'd thought.