Yesterday I noticed that grey gradient at the bottom of my Macbook Pro screen – it's the 2016 model and just 11 months old. For clarity in the screenshot I've hidden the dock and put a white background on.
It appears over the top of anything on the screen but due to its relevatively small size it's not (yet) a bother. I assume it's backlight issue of some kind but not sure what exactly would produce that kind of pattern – consistent right across the screen.
Thankfully my employers swap out our devices after 18 months, so I've only got to live with this (if I want to) for 7 months. The majority of the time I use an external monitor and only really use the laptop screen whilst mobile. But, still, it's a little odd!