Your advice helped on one of two problems.
The ~/.xinitrc file solved problem that X was only using little corner on display. After startx, the whole display was used. It gave me the happy memory of running Slackware in 1996, when that was how Linux always looked. Thanks.
On the gdm3 problem, I’m still failing. I changed the modeset value in /lib/modprobe.d/nvidia-kms.conf and ran the initramfs. After restart, as root, I run “systemctl start gdm3” and I still get black screen with 1 underscore in white at the top left. This is puzzling because there is no log file created in /var/log/gdm3. Are these permissions correct?
drwx–x–x 2 root gdm 4096 Oct 11 2017 gdm3