First of all, you must create an image, to use as mask, that is suitable for your needs. I would suggest you to create it in Gimp or another raster editing software since they're specifically created for these purposes.
Let's assume you already have your mask image created and exported as PNG with alpha channel like below:
Import your image into inkscape. For example purpose, I have created two version of the same mask, inverting the transparent area to show different results. As you can see, near the two versions of the image, there are two identical shapes filled with a gradient.
All you have to do is:
- ensure the image is ordered ABOVE the shape you want to mask (using these )
- drag the image mask above the shape (if you want to mask many shapes at once, group them first)
- select both image mask and the shape (or group)
- use menu OBJECT > MASK to apply the mask
And this is the final result: