Otdfbt

In gnome-terminal Ubuntu Unity 16.04 LTS the menu works for all three zoom features within the View menu:

• Zoom In


• Zoom Out


• Normal Size

Only two shortcut keys work:

• 
  Ctrl+Shift++ works to Zoom In
  


• 
  Ctrl+- works to Zoom Out.

Attempting to restore Normal Size I found Ctrl+o or Ctrl+Shift+O do not work. These key combinations perform as if the Enter was used.

My modified setup

$ cat .inputrc

# ~/.inputrc - complements /etc/inputrc - global inputrc for libreadline
# April 15, 2019
# See readline(3readline) and `info rluserman' for more information.

$include /etc/inputrc

# Insert key to togged overwrite-mode
"e[2~": overwrite-mode

# April 22, 2019 AU Q&A: https://askubuntu.com/q/1135306/307523
# Make Shift-tab act like "cd ../" (move to parent directory)
"e[Z": "cd ../"

Custom keyboard shortcuts

• I believe these are standard. At least I don't remember changing them.


• I reviewed all the other shortcuts and Ctrl+o doesn't appear.

Ubuntu skeleton setup

$ cat /etc/inputrc

# /etc/inputrc - global inputrc for libreadline
# See readline(3readline) and `info rluserman' for more information.

# Be 8 bit clean.
set input-meta on
set output-meta on

# To allow the use of 8bit-characters like the german umlauts, uncomment
# the line below. However this makes the meta key not work as a meta key,
# which is annoying to those which don't need to type in 8-bit characters.

# set convert-meta off

# try to enable the application keypad when it is called. Some systems
# need this to enable the arrow keys.
# set enable-keypad on

# see /usr/share/doc/bash/inputrc.arrows for other codes of arrow keys

# do not bell on tab-completion
# set bell-style none
# set bell-style visible

# some defaults / modifications for the emacs mode
$if mode=emacs

# allow the use of the Home/End keys
"e[1~": beginning-of-line
"e[4~": end-of-line

# allow the use of the Delete/Insert keys
"e[3~": delete-char
"e[2~": quoted-insert

# mappings for "page up" and "page down" to step to the beginning/end
# of the history
# "e[5~": beginning-of-history
# "e[6~": end-of-history

# alternate mappings for "page up" and "page down" to search the history
# "e[5~": history-search-backward
# "e[6~": history-search-forward

# mappings for Ctrl-left-arrow and Ctrl-right-arrow for word moving
"e[1;5C": forward-word
"e[1;5D": backward-word
"e[5C": forward-word
"e[5D": backward-word
"ee[C": forward-word
"ee[D": backward-word

$if term=rxvt
"e[7~": beginning-of-line
"e[8~": end-of-line
"eOc": forward-word
"eOd": backward-word
$endif

# for non RH/Debian xterm, can't hurt for RH/Debian xterm
# "eOH": beginning-of-line
# "eOF": end-of-line

# for freebsd console
# "e[H": beginning-of-line
# "e[F": end-of-line

$endif

Any clues?

The default shortcut for restoring the zoom in gnome-terminal, as in many other apps, is Ctrl + 0 (zero)

You can alter it in gnome-terminal's preferences, that is, neither in inputrc nor among the global desktop-wide shortcuts.