Gimp is available from the ubuntu repos already, so you don't need any ppa.
If you execute apt download gimp
it will download a deb file of gimp to the folder where you have terminal open (usually /home).
I recommend also to delete that ppa if you don't need it. Adding ppas is a security risk to your system. You can do so with sudo add-apt-repository --delete ppa:otto-kesselgulasch/gimp