GNOME is not considered especially lightweight desktop environment. In my experience, however, any computer with at least 1GHz Pentium and 256 MB RAM can be used to run GNOME desktop. You might consider it unusable, but I have noticed it is better to install GNOME to be used by my non-geek wife than to force her use some lightweight window manager that does not pop up icons for memory sticks on the desktop...
If you are looking for a lightweight Linux distribution with GNOME, you should seriously consider Zenwalk 5.2 with GNOME. Zenwalk has for years been a nice lightweight distribution that is based on Slackware. Thus it has been a delight to hack and tweak.
Read the release notes before deciding. The Zenwalk Gnome 5.2 iso weighs 597.7MB and can be downloaded from the following locations:
http://distro.ibiblio.org/pub/linux/dis ... me-5.2.iso
http://pnboy.pinguix.com/gapan/zengnome ... me-5.2.iso
http://zen-repo.meticul.eu/i486/zenwalk ... me-5.2.iso
http://ftp.nux.ipb.pt/pub/dists/zenwalk ... me-5.2.iso
http://zenwalk.linuxish.net/i486/zenwal ... me-5.2.iso
http://chronos.iut-bm.univ-fcomte.fr/di ... me-5.2.iso