Dear Maintainer,

First of all I tried editing /etc/gdm3/daemon.conf.
At the [greeter] section I placed:

....
[greeter]
IncludeAll=false
Include=myuser
....

To only include "myuser".

dpkg-reconfigure dm3
service gdm3 restart

And I'm presented with the same login list (all the users)

Then:

....
[greeter]
IncludeAll=false
Exclude=excludeduser
....

Same result.

And also the combination of two which gave the same result.

I also tried to change /etc/passwd and set for the users which
I didn't want to appear /bin/false as the login shell, but
that didn't work out either.

I know that if the id of the user is under 1000 it will not
get displayed, well, I can change id of the users, but that's
a bit hacky.