You might try configuring the gsetting for gdm background and see if that yields any luck:
sudo -u gdm dbus-launch gsettings set org.gnome.desktop.screensaver picture-uri 'file://(place path to file here'
For example; if your file is in /usr/share/pixmaps and is named space-oriented.png:
sudo -u gdm dbus-launch gsettings set org.gnome.desktop.screensaver picture-uri 'file:///usr/share/pixmaps/space-oriented.png'
Thanks for reply!
I tied it, set that up, also checked if saved successfuly, indeed it did, but the login screen still doesn't show my wallpaper. I mean, it's not a big deal, it's just an esthetic thing for me, but I'd be happier if it worked properly 
I understand as I am much the same way. For me, I use LightDM instead of GDM.
You can try that if you like:
sudo apt install lightdm
It will give terminal prompts and a configuration.
After that, you will want to install
sudo apt install lightdm-gtk-greeter-settings
I've found the default background image, swapped it for mine, but still it didn't change at login.
Is there an extra step I'm missing, to get it working?
Possible locations to check for Login-background:
/etc/alternatives
/usr/share/images/desktop-base
Not there, at least on Core Edition.
There appears to be a copy of the login-background in each gnome-shell directory. -Shrug- You might try just replacing all of them.
Already did that. Didn't help.
A quick web search I just did indicates that you did it right- I am sorry I am not much help, here. As I stated above, I always use LightDM as I find GDM to be stubborn and baulky.
Changing the background file resulting not changing the actual login background might indicate that its still present in some sort of "baked" module or cached somewhere else.
Will post my findings if any...
Honestly... My experience in such things on Linux desktops is that whenever we cannot change an icon or a setting somewhere, we often think it must be hard coded or inaccessible. Sometimes it is. But usually, when I find it, it was someplace really simple.
You might also look in /usr/share/backgrounds as well.
This time its more possible its baked into gnome-shell-theme.gresource file which every theme seems to have on Zorin OS.
I'll try to extract it and look for clues...
I actually just tried that too;) I opened the symlink in /usr/share/gnome-shell to take a look.
Here are the contents.
(You can notice a login-background.png file reference in it).
So now its just a matter of extracting it and recompiling it with our new background and upon restart it should work.
gresource list gnome-shell-theme.gresource
/org/gnome/shell/theme/assets/activities-dark.svg
/org/gnome/shell/theme/assets/activities.svg
/org/gnome/shell/theme/assets/calendar-event-dark.svg
/org/gnome/shell/theme/assets/calendar-event-today-dark.svg
/org/gnome/shell/theme/assets/calendar-event-today.svg
/org/gnome/shell/theme/assets/calendar-event.svg
/org/gnome/shell/theme/assets/checkbox-dark.svg
/org/gnome/shell/theme/assets/checkbox-focused-dark.svg
/org/gnome/shell/theme/assets/checkbox-focused.svg
/org/gnome/shell/theme/assets/checkbox-off-dark.svg
/org/gnome/shell/theme/assets/checkbox-off-focused-dark.svg
/org/gnome/shell/theme/assets/checkbox-off-focused.svg
/org/gnome/shell/theme/assets/checkbox-off.svg
/org/gnome/shell/theme/assets/checkbox.svg
/org/gnome/shell/theme/assets/dash-placeholder-dark.svg
/org/gnome/shell/theme/assets/dash-placeholder.svg
/org/gnome/shell/theme/assets/login-background.png
/org/gnome/shell/theme/assets/toggle-off-dark.svg
/org/gnome/shell/theme/assets/toggle-off.svg
/org/gnome/shell/theme/assets/toggle-on-dark.svg
/org/gnome/shell/theme/assets/toggle-on.svg
/org/gnome/shell/theme/gdm3.css
/org/gnome/shell/theme/gnome-shell.css
/org/gnome/shell/theme/icons/scalable/actions/color-pick.svg
/org/gnome/shell/theme/icons/scalable/actions/pointer-double-click-symbolic.svg
/org/gnome/shell/theme/icons/scalable/actions/pointer-drag-symbolic.svg
/org/gnome/shell/theme/icons/scalable/actions/pointer-primary-click-symbolic.svg
/org/gnome/shell/theme/icons/scalable/actions/pointer-secondary-click-symbolic.svg
/org/gnome/shell/theme/icons/scalable/status/eye-not-looking-symbolic.svg
/org/gnome/shell/theme/icons/scalable/status/eye-open-negative-filled-symbolic.svg
/org/gnome/shell/theme/icons/scalable/status/keyboard-caps-lock-filled-symbolic.svg
/org/gnome/shell/theme/icons/scalable/status/keyboard-enter-symbolic.svg
/org/gnome/shell/theme/icons/scalable/status/keyboard-hide-symbolic.svg
/org/gnome/shell/theme/icons/scalable/status/keyboard-layout-filled-symbolic.svg
/org/gnome/shell/theme/icons/scalable/status/keyboard-shift-filled-symbolic.svg
/org/gnome/shell/theme/icons/scalable/status/message-indicator-symbolic.svg
/org/gnome/shell/theme/no-events.svg
/org/gnome/shell/theme/no-notifications.svg
/org/gnome/shell/theme/pad-osd.css
/org/gnome/shell/theme/process-working.svg
Exactly- that is an external image file. So changing that image file should change it on the login screen.
Just for grins - what happens if you cycle through which gnome-shell theme you have enabled and test login on each?
Changing the theme does not help and its logical, cause no change was done to used *.gresource files.
I think *.gresource file of the theme of our use needs to get recompiled somehow with the current wallpaper as login-background.png.
I am on Zorin OS 16 so I cannot check easily- and zorin 15 (Ubuntu 18.04) handled the GDM3 and background a bit differently.
Can you check if you have a gdm3.css file in /usr/share?
It may show a direct url to where the image is.
Otherwise, for the gresource, you may find this application handy:
When I used Manjaro I always used "Loginized": You'll find it here: GitHub - juhaku/loginized: Gnome GDM Login Theme Manager. Easy and Fast Login Theme Manipulation
It's drop and drag.
Hi again 
The official blog says, that login screen now displays a blurred version of my wallpaper.
But still it does not blur any of custom set wallpapers.
Can someone close to the devs 
please check if custom wallpaper blur is actually supported in Zorin OS 16 Core (not talking about beta here anymore)?
The login screen isn't a blurred version of your wallpaper, but the lock screen are.
