Helping a new dev setup their machine today with rbenv when trying to bundle I ran into this error:

ERROR:  While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.


Thankfully I already dealt with this before and referenced it as a comment to a question on StackOverflow. For my own future reference I will list the solution here as well.

You need to correct your paths

To determine if this fix will work run the following:

which gem


This should output a directory you do not have permissions to:

/usr/bin/gem


To fix this perform the following steps.

1. Determine the path you need to copy to your profile:
rbenv init -


The first line of the output is the line you need to copy over to your profile:

  export PATH="/Users/justin/.rbenv/shims:${PATH}" #path that needs to be copied source "/usr/local/Cellar/rbenv/0.4.0/libexec/../completions/rbenv.zsh" rbenv rehash 2>/dev/null rbenv() { typeset command command="$1"
if [ "$#" -gt 0 ]; then shift fi case "$command" in
rehash|shell)
eval rbenv "sh-$command" "$@";;
*)
command rbenv "$command" "$@";;
esac
}

2. Copy the path to your profile and save it
4. Run rbenv rehash
Now when you run which gem you should get a local path that you have permissions to:
/Users/justin/.rbenv/shims/gem