I did a short video on a cool new Lightroom 6/CC feature that especially landscape photographers will love. But if you're more into shooting portraits, make sure to watch the video, too because the technique I describe works just as well with the Radial filter!
The Auto Mask feature I explain in the video works best if the area you're painting over is relatively similar in color and that colour is different enough from the background colour.
By the way, there's an even faster way to make custom graduated filters like this using Nik Software's Color Efex Pro. I explain all about that in this blog post which coincidentally features the same sample image!