Hi.
Thank you very much. After reading your post, I entered the command with -S (uppercase) and the output was
curl: (28) Resolving timed out after 10000 milliseconds
I had a look for information about the cause of curl: (28), and one of the potential causes is a poorly-configured dns server. The pihole diagnostics indicate pihole is working as a dns server for clients, but it isn't clear it tests for its own DNS. I tested that and it was not working for its own lookups.
The resolv.conf for the underlying Debian install on which pihole resides points to my router as the nameserver, which in turn provides the static ip address for the pihole machine as the DNS server. I thought this was the correct approach but that seems to have been an error on my part. I left the router as is, but changed the nameserver for the underlying Debian OS to a Quad9 option, and then to 127.0.0.1, and in each case, gravity updated fine.
I have left the nameserver for the pihole machine as 127.0.0.1 consistent with this older discussion.
So I think all is working ok now, but would appreciate any thoughts on if I am missing something. I do not fully understand why the original setting (namesever on Debian pointing to the router, as with other clients on the network) doesn't work. (I've read this, but it is not gelling.)
I ran another pihole debug (Token: https://tricorder.pi-hole.net/IdRD9Hlj/ ) and didn't see any issues timestamped after the change, but am obviously ignorant.
The good news, though, is things appear to be working.
Again, many thanks for the assistance so far (!) and any insight on why the other settings were not working would be much appreciated.