These are key bindings of the shell (bash), not of the terminal. See What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'? Nonetheless, the terminal is involved in translating keychords into escape sequences. See How do keyboard input and text output work?
Bash uses Emacs key bindings (unless you switch to vi mode). In Emacs, Alt+BackSpace (M-backspace) deletes the previous word. Alt+BackSpace (C-backspace) was traditionally not bound, though modern versions also bind it to the command to delete the previous word.
The escape sequence for Alt+key is the escape character followed by the character for key, when key sends a single character. There's no standard escape sequence for Ctrl+BackSpace, and it's usually not distinguishable from other keys. This is why terminal applications (including classic Emacs) tend not to have bindings for Ctrl+BackSpace. In gnome-terminal, Ctrl+BackSpace sends the same characte as BackSpace, so it isn't possible to have a separate binding for Ctrl+BackSpace.
Gnome-terminal's keychord-to-escape-sequence translations are not configurable. If you want to use different translations, use a configurable terminal emulator such as rxvt or xterm.
