uninstall_gems.sh

#!/usr/bin/env bash
#
# Originally from https://gist.github.com/IanVaughan/2902499
#
# authors: Ian Vaughan
#          Jacob Zimmerman
#
# usage:   uninstall_gems [<version> ...]
#
# examples:
#   Uninstall all gems in all ruby version
#       uninstall_gems
#
#   Uninstall all gems in ruby 2.1.3
#       uninstall_gems 2.1.3
#
#   Uninstall all gems in the current ruby version
#       uninstall_gems $(rbenv version-name)


uninstall() {
  list=`gem list --no-versions`
  for gem in $list; do
    gem uninstall $gem -aIx
  done
  gem list
  gem install bundler
}

if [ "$@" ]; then
  RUBIES="$@"
else
  #rbenv versions --bare
  RBENVPATH=`rbenv root`
  echo $RBENVPATH
  RUBIES=`ls $RBENVPATH/versions`
fi

# Don't clobber existing .ruby-version file
if [ -f ./.ruby-version ]; then
  RUBY_VERSION="$(cat ./.ruby-version)"
fi

for ruby in $RUBIES; do
  echo '---------------------------------------'
  echo $ruby
  rbenv local $ruby
  uninstall
done

# Restore old .ruby-version file if there was one
if [ "$RUBY_VERSION" ]; then
  echo "$RUBY_VERSION" > ./.ruby-version
else
  rm ./.ruby-version
fi

Here were the steps I used to run this command on my machine:

cd ~/code/github
git clone git@gist.github.com:cc2ba08062c6183a489c.git uninstall_gems
cd uninstall_gems
chmod +x uninstall_gems.sh
./uninstall_gems.sh

Works great, thanks!