With macOS Catalina, the setup instructions state to add the following path to your shell config
echo ‘export PATH="/usr/local/opt/ruby/bin:$PATH"’ >> ~/.bash_profile
Since zsh, Apple has renamed their config the .bash_profile to .zprofile
Should the shell config be:
echo ‘export PATH="/usr/local/opt/ruby/bin:$PATH"’ >> ~/.zprofile
Dave