You can install it with default yum install -y ruby ruby-devel rubygems

Other way is to compile it by yourself

Ruby

wget http://ftp.ruby-lang.org/pub/ruby/1.8/ruby-1.8.7-p302.tar.gz
tar -zxvf ruby-1.8.7-p302.tar.gz
cd ruby-1.8.7-p302
./configure --with-openssl-dir=/usr/lib/openssl
make
make install

Rubygems

wget http://production.cf.rubygems.org/rubygems/rubygems-1.3.7.tgz
tar -zxvf rubygems-1.3.7.tgz
cd rubygems-1.3.7
ruby setup.rb config
ruby setup.rb setup
ruby setup.rb install

Check if installed

ruby -v
gem -v