Using a gradient mask to blur part of an image

[spinsheet]

I have a photo and I would like to blur the bottom of the image while leaving the top of the image sharp, similar to limiting the depth of field by using a large f-stop. I created a gradient mask and then used Filters > Blur but it is not working. If I choose the layer that I created the mask on and choose blur, nothing happens. If I choose the layer (background) under the mask then the whole image blurs.

Can anyone offer a suggestion?

Thanks.

[spinsheet]

That worked great.

Thanks!

[noelb65]

Worked nicely for me too, thanks