I’m not sure where you get the name “app engine” from. Its not a term I’ve heard to describe technology for cross-platfrom development with.
Anyway, about cross development:
The UI toolkit GNOME uses, GTK, has initial support for Android now. Its still experimental and only available from the development version.
Other than that, GTK can be used for Windows and macOS as well (and is used, like by Inkscape or Gimp).
That aside:
Apple and XCode itself only provide macOS and iOS target, and Microsoft and C# to my knowledge only desktop. So not sure why you mention them.
The only cross-platfrom toolkits which do provide mobile and desktop on all major platforms which I know of are Flutter and web-based applications (think React-Native and Electron).