So, you've got an AMD GPU, and you want to use it on Debian Stretch. Well, you're actually in luck. Installing the open source AMD drivers on Debian Stretch may just be the easiest and simplest install process in the history of Linux graphics drivers. Anyone who tried to install the open source AMDGPU drivers when they first launched alongside AMD's Polaris cards will tell you that the process is long and painful, and they were right. Now, all of the packages required have entered the mainstream, and most are actually installed by default with your Debian Stretch installation.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-install-amdgpu-drivers-on-debian-9-stretch-linux
NOTE: SOME OF THE POSTS BELOW ARE TRANSFERRED FROM LINUXCONFIG DISQUS COMMENTS.