Well, my use case was to be able to build for Android from Gnome Builder, regardless of whether Android implements GTK. So I thought, why not just mimic a GTK-like API and then enable the creation of GTK-lookalike GUIs for Firefox on Android using HTML/CSS/JS? Then we could have a GTK-React or whatever.
Judging from the development of, for example, Xamarin, React Native, and Kotlin Multiplatform, I would speculate that this project may not have enough resources to support a fully native implementation. On the other hand, WebGL and WebGPU have become very performant.
I would be happy just with a GObject and GTK mapping to JavaScript and the Firefox API on Android.
However, there is already a thread for this: Why not a GNOME "app engine"? - #2 by CodedOre