Since Ubuntu 18.04 is a LTS release it will be officially supported by AMD with the AMDGPU-PRO drivers. These drivers are only available directly from AMD and are geared more towards professional applications. You don't need these drivers for gaming. Actually, the "gaming" part is almost identical to the open source driver.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-install-the-latest-amd-radeon-drivers-on-ubuntu-18-04-bionic-beaver-linux