Bug#655328: gdm3: No, your mistake.

[jaakov at ro.ru]

This time it's your mistake, my friend.
The file /etc/gdm3/greeter.gconf.defaults is not present on my system.
However, /etc/gdm3/greeter.gconf-defaults is present.

The corrected suggestion of Josselin for completely removing the 
background worked on at least one computer.
[org.gnome.desktop.background]
picture-options='none'
primary-color='#000000'

As soon as I try it on another computer, I'll report here. To the best 
of my knowledge, the documentation for this feature is still missing for 
debian, which made the feature unusable for the vast public.