Help on layer mask

[MrGimp]

Hi - I'd like some help on understanding layer masks. 

I want to blend an image that has white clothing on one edge into a white background so there is a seamless transition (the whites are not quite the same, leaving a sharp edge.

I get the effect I want from using layer mask and the graduation tool, but I feel I am missing a step as the effect doesn't "stick" - i.e. it goes back to the original as soon as I move on.

Can anyone help?

Thanks
MrGimp