Where do gems install?

Look at your gem environment.

In a terminal run gem env

You should see an entry INSTALLATION DIRECTORY, but there is also GEM PATHS which is where it's loading all your gems from in your current environment.

Rvm

$ rvm gemdir

Or you can check:

echo $GEM_HOME

Bundler

$ bundle show --paths

For specific gem:

$ bundle show 'gem_name'

Gem

$ gem env

For specific gem:

$ gem which 'gem_name'

To see the default installation directory, run

gem env gemdir

If you want to change the default installation directory (for example, to ~/.gem/ruby/2.1.0), add this line to ~/.bashrc

export GEM_HOME=~/.gem/ruby/2.1.0

And you also need to make sure ~/.gem/ruby/2.1.0/bin is in your PATH environment variable to use the commands provided by gem packages. If not, add this line to ~/.bashrc

export PATH=$PATH:~/.gem/ruby/2.1.0/bin