Ruby can be confusing to install. Make sure to plan out if per-user environments are going to be used, or system-wide (Ruby, Gems, etc installed by root).
Instead of using rvm or rbenv, stick to installing Ruby system-wide with pacman.
Caution editing your .bashrc or .zshrc.. root does not need to have a GEM_HOME or ~/.gem/ruby/2.0.0 added to its PATH
For root, create .gemrc so that when gems are installed as root, it uses the system-wide directory.
Optionally, create user's .gemrc as well, although it'd be the same as in /etc/gemrc, which is automatically made by the ArchPkg
Update system: sudo gem update –system Update user: gem update