Problems in starting GNOME

[Raffaele Borrelli <lello76 freemail it>]

Hello,
I think I have all GNOME files but I don't know
how to start a GNOME session.
I read the instruction reported of the GNOME guide, but
I don't know where are the binary files needed to run
gnome (I don't know which they are).
Many of the gnome files are in the directory opt/gnome/
and in the subdirectories therein, so I added the opt/gnome/ in the
path, and I added in my .xinitrc
the line 
exec gnome-session
(the only exec line)
but when I run startx the system can't find gnome.
Where should I look?