gcc-4.7.2 was released 3 days ago on September 20th. It won't have made it into the debian repos yet (See update2, thanks derobert).
According to the release notes it is a bug-fixing release so will probably use the same library versions as the previous, 4.7.1, release. So, since 4.7.1 is in the repos, install it to get all the dependencies sorted, then if you really really need the latest version, download the source and compile following the instructions from the gcc website.
UPDATE:
You seem to have a problem with your source.lst. As a workaround, try downloading the package here and installing using dkpg -i gcc_4.7.1-1_amd64.deb.
UPDATE 2:
As @derobert points out below, 4.7.2 is indeed in the experimental repo. So, adapt the instructions from the post you linked to:
Your /etc/apt/sources.list should look something like this:
deb local.debian.mirror squeeze main
deb local.debian.mirror unstable main
while your /etc/apt/preferences should look something like this:
Package: *
Pin: release n=squeeze
Pin-Priority: 900
Package: *
Pin: release n=unstable
Pin-Priority: 200
Then install using apt-get install gcc-4.7/unstable.
As both jordanm and the original tutorial you linked to mention, this is not a very good idea. Make sure to point your sources back to stable once you're finished.
At the configure the package manager step you should select (in top of the country mirror list):
enter information manually
Then select a debian mirror from here , for example:
archive.debian.org
Then the Debian mirror directory:
/debian/
You should ignore the next error saying :
security.debian.org couldn't be accessed
because there is no security updates for debian Lenny.
Best Answer
You just want some mirror or the closest/fastest mirror. If it's the latter, then you could just install netselect-apt and run it. I just ran to see which are the fastest form my geographical location and it said -
Hope you find it useful.