From where does "apt-get install package_name" download packages?
Solution 1:
Type:
apt-cache policy package_name
And it'll tell you. On Ubuntu, it's usually archive.ubuntu.com, but you may have other sources set up.
Solution 2:
cat /etc/apt/sources.list
cat /etc/apt/sources.d/*
should show you the repos used. I don't know how rubygems.org was protected but here is some useful pieces of advice on apt: http://wiki.debian.org/SecureApt it's for debian but it also applies to Ubuntu which uses the same packet manager.