This has been known for years. Referring to this bug report from 2007 it says:
comment from upstream:
"There are a couple of keys which are not candidates to be keyboard
accelerators, independently of the modifiers. Tab is one of those
keys, as it (with various combinations of modifiers) are reserved by
gtk for its own use and the window manager.
In other words, for
gnome-terminal Tab is a special key and you cannot use it as part of a shortcut.
The alternative I can suggest is to assign Ctrl + Tab to the
xdotool key Ctrl+Page_Down command.
xdotool is a program that allows simulating mouse behavior,key presses, and window controlling functions (very very versatile tool). I have used it quite a lot, so by this point if it's installed by default, so you will need to install it with
sudo apt-get install xdotool. Basically you will be pressing Ctrl + Tab but
xdotool will simulate the Ctrl + Pg_Down. Same idea with the reverse cycling shorcut.
The big disadvantage is that this is a global action, meaning that if some other program has Ctrl + Tab shortcut that isn't for tab switching, it will definitely conflict with that program. Another disadvantage is that you cannot hold Ctrl and keep on pressing Tab repeatedly. Only the whole shortcut works and once at a time.