How to disable the alt-hotkey behavior on gnome terminal?

Edit > Keyboard Shortcuts..., and uncheck "Enable menu access keys".

For Gnome terminal 3.14.1: Click Edit (on menu bar) > Preferences > At last on "General" tab, Un-check "enable mnemonics..." .

In Ubuntu 16.04, go to System Settings -> Keyboard -> Shortcuts -> Launchers, and clear the Key to show the HUD (or change it to another key)