A FAQ do terminal do GNOME declara:
How can I make the terminal transparent?
Since version 3.7 (NixOS master currently contains version 3.26) this option has been removed from the Preferences dialogue. You can however still get the same effect by setting the _NET_WM_WINDOW_OPACITY X property, for example with the Devil's Pie or Devil's Pie II tools. E.g., set up Devil's Pie to start automatically with the session, and create the file ~/.devilspie/gnome-terminal.ds with these contents:
(if (matches (window_name) "gnome-terminal-window-*") (opacity 90) )
You can also use this shell script that however only works for existing terminal windows and not automatically for newly created ones. - https://wiki.gnome.org/Apps/Terminal/FAQ#How_can_I_make_the_terminal_transparent.3F