I will add just one thing to this very valuable thread.
To verify if GPS is used by chrony you can run

chronyc sources -v
If not, check /etc/chrony/chrony.conf file and make sure that the line
refclock SHM 0 offset 0.5 delay 0.2 refid GPS
is uncommented (no # in front of the line).
If you need to edit the file, save changes and remember to restart chrony by running
sudo systemctl restart chrony