Is gnome built on top of twm?

[Foss User <fossist@gmail.com>]

Please see the following output I generated from Squeeze.

$ aptitude why twm
i   gnome                     Depends  gnome-desktop-environment (= 1:2.24.3~2)
i A gnome-desktop-environment Depends  gdm (>= 2.20.9)
i A gdm                       Depends  gnome-session |
x-session-manager | x-window-manager | x-terminal-emulator
i   twm                       Provides x-window-manager

1. It shows that twm was installed because gdm needs a window manager
and twm is one such window manager. Does it mean that gnome is
dependent on twm?

2. Is it possible to run gnome with some other window manager?

3. I am unable to see any twm process in the ps listing. Why?

4. Can someone share the equivalent 'aptitude why' output from a
system running KDE?