there's /usr/share/gnome-session/sessions/gnome-flashback.session copy it, rename the new file to gnome-compiz.session, edit this file, replace metacity with compiz.
also copy and rename the /usr/share/xsessions/trisquel-desktop.session edit it and replace "Exec=gnome-session --session=gnome-flashback" with "Exec=gnome-session --session=gnome-compiz"
