GNOME Pro Tip: The current GNOME default terminal (Ptyxis) supports transparency but doesn’t expose UI controls to adjust it.

# If using distro packaging
gsettings set org.gnome.Ptyxis.Profile:/org/gnome/Ptyxis/Profiles/$PTYXIS_PROFILE/ opacity .85

# If using app.devsuite.Ptyxis from Flathub
flatpak run --command=gsettings app.devsuite.Ptyxis set org.gnome.Ptyxis.Profile:/org/gnome/Ptyxis/Profiles/$PTYXIS_PROFILE/ opacity .85

Check out other details at https://gitlab.gnome.org/chergert/ptyxis.