I have researched the heck out of this question and found two pages about the issue but not clarifying it.
In the debian-installer during the optional software selection phase you have these options:
Debian desktop environment (already ticked by default) ... GNOME (not ticked) ... xfce (not ticked) ... KDE (not ticked) ... Cinnamon (not ticked) ... MATE (not ticked) ... LXDE (not ticked)
What does Debian desktop installer actually install? Does it install a GUI (Gnome, my understanding, is the default) or does it just install a handful of programs useful for desktop users but which do not include a GUI? Do you have to tick off Gnome to get the GUI or not? And if not, what is the purpose of the option to tick off Gnome in addition to Debian Desktop Environment?
The page concerning Desktop Environments in the Debian Wiki does not clarify the issue.
This thread on the Debian User Forums concerns this very issue but has a raft of contradictory answers.