(The Reply button on my yesterday's post doesn't work.)
I tried rebooting a few times, and finally noticed "Run UEFI application" in the boot menu on my HP machine. A number of menu choices later, and voila! It worked.
This seems an awkward way to get the installation bootup to understand it's dealing with a GPT disk, but what do I know? I've been fuddling with computer stuff since only 1962,