Where is the gem command in RHEL6?

You probably have to install RubyGems. Try:

yum install rubygems

If it fails, try installing it by hand:

wget http://rubyforge.org/frs/download.php/74922/rubygems-1.8.4.tgz
tar xzpvf rubygems-1.8.4.tgz
cd rubygems-1.8.4
ruby setup.rb