The GTK demo has an Android app build in their CI. You can see how to get the APK from that from a comment I made here.
Looking at the configuration of the CI, it seems that the heavy lifting of building the APK is done by a separate tool called pixiewood, with a few patches to GLib and meson.
As for GTK itself, most of the Android-specific code seems to be in the GDK backend for Android, which is then used normally by GTK.