$ env | grep PATH
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

$ ls /usr/local/bin
erb  gem  grunt  irb  node  rake  rdoc  ri  ruby  testrb

$ which -a gem
/usr/local/bin/gem

$ gem
-bash: /usr/bin/gem: No such file or directory

$ /usr/local/bin/gem
RubyGems is a sophisticated package manager for Ruby.  This is a
basic help message containing pointers to more information.