Following on from my Installing Arch Linux with KDE post, installing GNOME instead of KDE is very simple.
When editing the /etc/inittab file from the KDE guide, uncomment the gdm line and not the kdm one
# Example lines for starting a login manager #x:5:respawn:/usr/bin/xdm -nodaemon x:5:respawn:/usr/sbin/gdm -nodaemon #x:5:respawn:/usr/bin/kdm -nodaemon #x:5:respawn:/usr/bin/slim >/dev/null 2>&1