My environment is an Ubuntu 18.04.3 LTS wíth a zsh shell and an 'Oh My Zsh' framework. All commands in git work perfectly, but when I try to run the git repository browser: gitk, in my local repository, in order to see branches graphically, it appears "zsh: command not found: gitk". I don't know if I have to configure it somehow.
gitk
is a seperate tool. Install it using:
sudo apt-get install gitk