[rubygems] You don't have write permissions for the /var/lib/gems/2.3.0 directory

Ubuntu 20.04:

Option 1 - set up a gem installation directory for your user account

For bash (for zsh, we would use .zshrc of course)

echo '# Install Ruby Gems to ~/gems' >> ~/.bashrc
echo 'export GEM_HOME="$HOME/gems"' >> ~/.bashrc
echo 'export PATH="$HOME/gems/bin:$PATH"' >> ~/.bashrc
source ~/.bashrc

Option 2 - use snap

Uninstall the apt-version (ruby-full) and reinstall it with snap

sudo apt-get remove ruby
sudo snap install ruby --classic