Thanks very much for your help and your patience.
Noob that I am, I did not notice in the printer properties that the "enable" box was not ticked. It is ticked now, and the test page shows that CUPS+gutenprint worked with this printer. (I have no idea how that box became un-ticked. I don't go to printer properties often.)
Also, FYI, the Canon page you provided does not provide a link to linux drivers. The page detected that my OS is "Linux 64bit". When the driver button is pressed, the page gives this message "There is no driver for the OS Version you selected. The driver may be included in your OS or you may not need a driver."
So glad that it was a simple fix.
Grateful for your time.