It is not easy to make a different zoom level default. The easiest option to start with larger fonts is to open the Preferences within Gnome-terminal, then on the "text" tab of the default profile ("Unnamed" by default), select a custom font with the desired size.
Apart from the shortcut keys, the only alternative option to have a different zoom level is to pass the option on the command line needed to start the terminal, e.g.
gnome-terminal --zoom=1.4
One way to make this option persistent is to create a small wrapper script gnome-terminal in a directory that comes earlier in the search PATH, e.g. in your ~/.local/bin folder. If you do not have that directory, create it, and create a script gnome-terminal there with the content:
#!/bin/bash
/usr/bin/gnome-terminal --zoom=1.4 "$@"
Make the script executable.
Log out then back in. That will cause ~/.local/bin to be prepended to your $PATH. The command gnome-terminal now will point to your wrapper script, which will in turn call the "true" executable, passing the --zoom option and any other options.
Another option, but this will work only when launching the terminal from the applications overview, is to copy the .desktop launcher to your ~/.local/share/applications directory, and edit the Exec= line to include the --zoom option. Also an alias can be defined, but that will only work to automatically pass the option from an interactive terminal.
