On my laptop (Xubuntu 18.04) in the gnome-terminal the commands
gnome-terminal gnome-terminal --tab
both open a new tab.
On my desktop (Xubuntu 16.04) they both open a new window. Changing the option "Open new terminals in Tab/Window" under "Edit > Preferences > General" does not affect the behavior (on the desktop). I want to open tabs, what to do?