Note

This no longer works with more recent versions of OS X, including 10.10 Yosemite (I'm not sure about 10.9 Mavericks). It does work with 10.8 Mountain Lion, however.

It's actually not too hard, but you do need to have admin privileges (using the sudo command to write to /etc).

1. From Terminal (or your favorite substitute), see if there's anything in the file /etc/launchd.conf:

   cat /etc/launchd.conf
   

   If you get an error like

   cat: /etc/launchd.conf: No such file or directory

   then continue with the next step. If the cat command does display some content, copy it.

2. Determine your system's current path, as we'll need to make sure we include it later:

   launchctl getenv PATH
   

3. In your favorite editor, create a new text file with the following content, modified to fit your needs:

   setenv PATH /usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin:/Users/YourUserName/bin:/path/to/gems/bin
   

   Make sure you've included the entire contents of the path from the previous step, otherwise you'll break your system.

   If the cat command from Step 1 displayed some content, paste it into the new file before the setenv PATH command. If it already contains a setenv PATH command, just modify it to add the extra directories you need.

4. Save the new file in your home directory (/Users/YourUserName) as launchd.conf.

5. Go back to Terminal and enter:

   sudo mv ~/launchd.conf /etc
   

   to use admin power to move the new file to /etc, replacing anything that was there before. Depending on your previous usage of the sudo command, you may get a short "be careful doing what you're doing" message, but either way you'll need to enter your password. /etc is not directly accessible through the Save dialog of graphical editors unless you're a real power user and know how to get around OSX's file system restrictions.

6. Reboot your computer

And you should be all set. If you're interested, launchd and launchctl use the csh/tcsh syntax, so you can't use the bash/zsh export PATH=/usr/local/bin:... format.

command -pv uses a "default value for PATH".

$ which ruby
/home/mikel/.rvm/rubies/ruby-1.9.3-p484/bin/ruby

$ command -pv ruby
/usr/bin/ruby

Unfortunately that doesn't work in zsh, so based on Stephane's comment, we could use getconf PATH:

$ PATH=$(getconf PATH) which ruby

or use command -v in place of which, as recommended in Why not use "which"? What to use then?

$ PATH=$(getconf PATH) command -v ruby

The downside with these approaches is that if the system administrator installed a system-wide version into say /usr/local/bin or /opt/local/bin, and all users had that in PATH (e.g. via /etc/profile or /etc/environment or similar), the above probably wouldn't find it.

In your specific case, I'd suggest trying something specific to Ruby. Here's some ideas that might work:

1. Filter out versions in the user's home directory (and relative paths):

   (
       IFS=:
       set -f
       for dir in $PATH; do
           case $dir/ in
               "$HOME/"*) ;;
               /*/)
                   if [ -f "$dir/ruby" ] && [ -x "$dir/ruby" ]; then
                       printf '%s\n' "$dir/ruby"
                       break
                   fi;;
           esac
       done
   )
   

2. Filter out versions in the rvm directory

   (
       IFS=:
       set -f
       for dir in $PATH; do
           case $dir/ in
               "$rvmpath/"*) ;;
               /*/)
                   if [ -f "$dir/ruby" ] && [ -x "$dir/ruby" ]; then
                       printf '%s\n' "$dir/ruby"
                       break
                   fi;;
           esac
       done
   )
   

3. Filter out writeable rubies (last resort, assumes not running as root)

   (
       IFS=:
       set -f
       for dir in $PATH; do
           case $dir/ in
               /*/)
                   ruby=$dir/ruby
                   if [ -f "$ruby" ] && [ -x "$ruby" ] && [ ! -w "$ruby" ]; then
                       printf '%s\n' "$ruby"
                       break
                   fi;;
           esac
       done
   )
   

4. Ask rvm, chruby, etc.

   (
       rvm system
       chruby system
       command -v ruby
   )
   

The last way makes rvm select the default Ruby, but we do it in a subshell, so that the user's preferred Ruby is restored afterwards. (chruby part untested.)