Gradient blur

I tend to agree that the changes to the Gradient tool are not as intuitive as they used to be.  They may be more powerful, but a lot of the simple ways we used gradients, and running a grdient down a layer mask is the perfect example, are not as quick and easy as they used to be till you get a handle on it.

What I suggest you do is before you go into Quick Mask, open the new Gradients panel, and click on the menu (top right) to load the legacy gradients.

...where you'll find our old favourites like Foreground to transparent.  If they are not in a convenient location drag the group to the top..

In fact, better still, make a new group and drag your favourite presets to it

There's a wee trick to the next bit, but we have already agreed this was not so easy now.

With black as the foreground colour, click on the gradient tool, and having already selected foreground to transparent, click on the gradient in the options bar and it will show what you expect. 

However, swapping to white as the foreground colour does not change the gradient. It still shows black to transparent. You have to click on the gradient in the options bar to open the drop down where'll find the correct gradient.

So finally getting to your question.  Use your old method with Quick Mask and run the graient (black to white works the same as black to transparent, except you can add to foreground to transparent).  Come out of Quick Mask and make the layer a Smart Object.  You can now use the SO mask to control your efects.

Note: the screen shot just shows Blur Gallery, but I used Field Blur as — unlike Gaussian blur — Field blur can be edited so is non-destructive

with "Blue" I meant "Blur"