Interesting, I didn't know about those services. I tried to add a URL and I just got the same error, that is, 'The value entered is not valid', with no explanation of what exactly would make it valid.
Do you know about the GINA web layers github repo, from which I got the address I was trying?
GitHub - gina-alaska/gina-map-layers: Layer definitions and apis for accessing GINA map tiles
Update: I think I finally found the problem. On this page - Choose global or local—ArcGIS Online Help | ArcGIS - there is the explanation that tile layers have to be published from an ArcGIS server, which is annoying because when I was at the developer summit and mentioned to an ESRI person that I had tried using the GINA tile layers with Cesium, he asked 'Why would you use Cesium?'. I didn't have a great answer at the time, but now I do.