Try nathanwhy's answer before using my original answer below. His recommendation of --user-install should accomplish the same purpose without having to muck with your .bash_profile or determine your rubyRuby version.
If you are not concerned about a specific ruby version, you can skip the heavy-lift rubyRuby environment manager options, and just add these lines to ~/.bash_profile:
export GEM_HOME="$HOME/.gem/ruby/2.0.0"
export GEM_PATH="$HOME/.gem/ruby/2.0.0"
The path is stolen from the original output of gem env:
RubyGems Environment:
- RUBYGEMS VERSION: 2.0.14
- RUBY VERSION: 2.0.0
- INSTALLATION DIRECTORY: /Library/Ruby/Gems/2.0.0
- RUBY EXECUTABLE: /System/Library/.../2.0/usr/bin/ruby
- EXECUTABLE DIRECTORY: /usr/bin
- RUBYGEMS PLATFORMS:
- ruby
- universal-darwin-14
- GEM PATHS:
- /Library/Ruby/Gems/2.0.0
- /Users/mylogin/.gem/ruby/2.0.0 # <---- This guyline, right here. -----
- /System/Library/.../usr/lib/ruby/gems/2.0.0
...
No sudoing is required, and you can use the already-installed rubyRuby, courtesy of Apple.