I have '12h' set in /org/gnome/desktop/interface/clock-format, but that is not obeyed and the 24-hour clock is shown.
Am I missing something?
Hey 
Normally, I switch the AM/PM or 24h clock on GNOME Settings itself
But it works once logged in?
GNOME Settings sets preferences for your user account; those are not used by GDM, which has its own distinct set of preferences. On Fedora systems those live in /var/lib/gdm.
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Thanks, all… Changing the system’s 12/24-hr option in GNOME Settings only changes it for the session user and I should have been more specific that I used sudo dconf-editor to make the changes and that should have made the root changes I’m looking for; however, for what it’s worth, /var/lib/gdm is empty for me.
I do, though, have an /etc/gdm folder, but there doesn’t seem to be anything interesting in is other than a file named custom.conf that’s practically empty.
P.S., I can also verify 12-hr format selected in GNOME Control Center after running sudo gnome-control-center.
I think that making this change with sudo/root doesn’t matter, since GDM runs as the gdm user if I’m not wrong.
That actually makes sense… Any suggestions on how to make those changes? It’s trivial, for sure, but irritating that I can’t figure out how to make it happen…
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Try this software
https://flathub.org/apps/details/io.github.realmazharhussain.GdmSettings
Exactly, irritates me to if i need an extension to install extensions, and then i need to know which extension to change which applications or part of the OS i can change with.
But in the end I do like it very much, how fast and with which enthusiasm, we do get help here on solving such tricky tasks. Thx @alciregi for the link to GdmSettings. Now I can change also the background as I could with lightDM.
My conclusion:
We do need a new extension to manage our important extensions 
Something like the installer for the “everything Fedora” installer.
https://docs.fedoraproject.org/en-US/quick-docs/displaying_user_prompt_on_gnome_login_screen/
Started with that for a reference
The key file “01-desktop-interface”, looks like:
[org/gnome/desktop/interface]
clock-format=“24h”
So it looks like either installing an extension or going through the motions you suggest, @grumpey…
Thank you to everyone for the help!
EDIT
For anyone who wanders along, I tried out the extension and it seems to have worked with one caveat: when you apply the changes, it completely screwed up my desktop appearance until restarting the laptop.
I’ll mark the extension as the “solution,” but only because I didn’t attempt the changes suggested by Joe.