Hello!
While I'm aware that applications like Story Maps allow for the inclusion of custom Google Fonts (see Image 1), I haven't been able to locate a similar configuration in Map Viewer. It woud be great if there's a method or workaround for incorporating a new Google Font into Map Viewer's font selections. Any suggestion on this matter would be well appreciated!
Would this older community post help?
Custom label in New Map Viewer - Esri Community
Not sure if you're using Online or Portal, so especially notice the reply given by @RussRoberts at 04-20-2023 05:26 AM concerning the release dates.
Hope this helps.