I have never had a default installation of Ruby gems on Mac OS X let me simply install new gems without requiring sudo. If you would like to install Ruby gems without requiring sudo, do the following:

1. Add GEM_HOME to your .bash_profile

   For example, nano ~/.bash_profile and add export GEM_HOME=/Users/Michael/.gem where the path is to your own Home folder

2. Add the gem executables to your system path

   Also in .bash_profile, add export PATH="$GEM_HOME/bin:$PATH"

For some reason these simple steps don’t show up in the top StackOverflow answers. People often suggest using rvm to install a custom Ruby version that won’t require using sudo when installing gems or something like rbenv to create a Ruby environment for the same benefit. If you simply want to use the default installed Ruby version on Mac OS X (this works in El Capitan) without requiring sudo, this should do it.