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- First method
Add this line to ~/.bashrc:

- Second method
Use update-alternatives:

$ which editor
editor is /usr/bin/editor
$ sudo update-alternatives --install "$(which editor)" editor "$(which vim.gtk3)" 15

- Third method
Set the EDITOR at runtime:

First method

Add this line to ~/.bashrc:

Second method

Use update-alternatives:

$ which editor
editor is /usr/bin/editor
$ sudo update-alternatives --install "$(which editor)" editor "$(which vim.gtk3)" 15

Third method

Set the EDITOR at runtime:

First method, you can add this line to ~/.bashrc:

Start new bash session to take effect, then run sudo systemctl edit <foo> as usual.

Second method is use update-alternatives:

Third method is direct set the EDITOR on runtime:

The precedence are first method > third method > second method.

- First method
Add this line to ~/.bashrc:

Start a new bash session to take effect, then run sudo systemctl edit <foo> as usual.

- Second method
Use update-alternatives:

- Third method
Set the EDITOR at runtime:

The order of preference is 1st method > 3rd method > 2nd method.

You can add this line to ~/.bashrc (gedit as example, since my default is already vim.gtk3):

export SYSTEMD_EDITOR=gedit

Second way is use update-alternatives:

Install your desired editor, e.g. gedit:

$ type -a editor
editor is /usr/bin/editor
editor is /usr/bin/editor
$ sudo update-alternatives --install /usr/bin/editor editor "$(which gedit)" 5

$ sudo update-alternatives --config editor
There are 7 choices for the alternative editor (providing /usr/bin/editor).

  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /usr/bin/vim.gtk3    50        auto mode
  1            /bin/ed             -100       manual mode
* 2            /bin/nano            40        manual mode
  3            /usr/bin/code        0         manual mode
  4            /usr/bin/gedit       5         manual mode
  5            /usr/bin/vim.basic   30        manual mode
  6            /usr/bin/vim.gtk3    50        manual mode
  7            /usr/bin/vim.tiny    15        manual mode

Press <enter> to keep the current choice[*], or type selection number: 4
update-alternatives: using /usr/bin/gedit to provide /usr/bin/editor (editor) in manual mode

Third way is direct set the EDITOR on runtime:

sudo EDITOR=gedit systemctl edit <foo>

First method, you can add this line to ~/.bashrc:

export SYSTEMD_EDITOR=vim

Second method is use update-alternatives:

Install your desired editor, e.g. vim.gtk3:

$ which editor
editor is /usr/bin/editor
$ sudo update-alternatives --install "$(which editor)" editor "$(which vim.gtk3)" 15

$ sudo update-alternatives --config editor
There are 7 choices for the alternative editor (providing /usr/bin/editor).

  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /usr/bin/vim.gtk3    50        auto mode
  1            /bin/ed             -100       manual mode
* 2            /bin/nano            40        manual mode
  3            /usr/bin/code        0         manual mode
  4            /usr/bin/gedit       5         manual mode
  5            /usr/bin/vim.basic   30        manual mode
  6            /usr/bin/vim.gtk3    50        manual mode
  7            /usr/bin/vim.tiny    15        manual mode

Press <enter> to keep the current choice[*], or type selection number: 6
update-alternatives: using /usr/bin/vim.gtk3 to provide /usr/bin/editor (editor) in manual mode

Third method is direct set the EDITOR on runtime:

sudo EDITOR=vim systemctl edit <foo>

The precedence are first method > third method > second method.

Don't try to set "GUI" editor such as gedit because Why don't gksu/gksudo or launching a graphical application with sudo work with Wayland? and Gedit uses 100% of the CPU while editing files