Yesterday I tried it for a few hours without success.
First I tried with "Source Pro Serif", then I tried with a simpler name like Ubuntu. But that fonts have never been loaded from web resources. Indeed I do not really intend using web fonts, as I prefer using the locale fonts which a user has installed already, but I tried to learn how I would do it if necessary.
Finally I wondered if fail is related to the fact that https://fonts.google.com/ when called replies after a few seconds with message "Something went wrong. Please try again soon."
Funny fact: My Firefox browser seems to block Google Web Fonts. That is surprising as it is latest Firefox 74 and I have never modified its settings, so it should use default setting. Maybe default is blocking?
With the Gnome web browser Gazette them is displayed fine!