It is pretty easy to install Ruby on Debian. Just execute this as root:
apt-get install ruby1.9.1
That’s it! Depending on that GEMs you want to install you should install this here, too.
apt-get install make bcrypt libxml-ruby1.9.1 libxml2-dev libxslt-dev libpq-dev g++
Your GEMs are at “/var/lib/gem/1.9.1/”. You should add this here your path: