as far as I know, gnome isn't a part of the iso disc you can download from Arch's site, so that it isn't on the disc you install Arch from (correct me if I'm wrong or this is outdated). Gnome can be obtained with pacman through the Extra package reposity, and it's easy as eating an icecream:
$ pacman -Ss gnome
gives you (among others) the following:
extra/gnome-desktop 2.4.1.1-1
The GNOME Desktop
this means, that by making pacman get "gnome-desktop" it solves the dependencies and gets all the packages you need for gnome, and then installs them - of course, asking you whether you want to get the packages it needs (a list is shown) or not, before starting the actual operation.
the "magic" command switch is S - it tells pacman to search for the given package, check what it needs, list them, ask your opinion, download and install. example:
$ pacman -S gnome-desktop
and there you go. just remember, if you want to get the latest, to do a
$ pacman -Sy
from time to time, to keep your databases updated. and if you wish to be able to get development (unstable) versions too, just edit pacman's config file to accept these too (the Unstable reposity).
I haven't myself got gnome, because I prefer fluxbox to it...but dependency-solving is a part of pacman's job, so whatever the gnome desktop needs, should be pulled from the net and installed by one command. that's one of the sides I like in Arch...RH was a true dependency-hell for me back in the days
and yes, Arch has proven to be the best distro for me, at least for now. and altough it's young, it keeps on developing and becoming better and better... I've got nothing big to complain about, and all the small things I do, are being fixed quickly as new versions come