This should do it: http://ubuntuhandbook.org/index.php/2018/04/how-to-install-gimp-2-10-in-ubuntu-18-04-17-10-via-ppa/
it works in Ubuntu but not elementary OS 5 beta. Maybe the final version will have the new gimp added.
I get this error... davek@optiplex780:~$ sudo add-apt-repository ppatto-kesselgulasch/gimp [sudo] password for davek: sudo: add-apt-repository: command not found davek@optiplex780:~$
You need to install the software that allow you to add PPAs. Code (Text): sudo apt-get install software-properties-common