We will assume that you have installed Gnome and the related packages. This includes the Gnome Display Manager (gdm) which, once installed, will automatically start up when you boot into Debian.
The essence to running Gnome is to include the following in the file /.xsession:
exec gnome-session
Alternatively if you are using the Gnome Display Manager (gdm) then select Gnome or Debian from the Session menu.