It's an unofficial repository (PPA) for Nvidia drivers on Ubuntu and Mint: https://launchpad.net/~graphics-drivers/+archive/ubuntu/ppa
If you add "ppa:graphics-drivers/ppa" in "Software Sources", you'll be able to install newer driver versions in the "Driver Manager". For a 50-series GPU, you'll want at least version 570 IIRC.
The PPA just provides the packages, you can actually install them through the Driver Manager after adding the PPA. However, without the PPA, the newest available version seems to be 550, which is not new enough for a 50-series GPU.