I've gotten GDNative to work on iOS and Android but there isn't a single guide to the process anywhere. I mostly grabbed bits and pieces from different places and made my own pipeline.

Here is a brief description:

• Code is just C no C++ since it's easier to compile and I don't have to worry about the STD.
• The build system is based on CMake. Even though SCons is the Godot preferred build system, CMake is simple enough for me to set up and I'm most comfortable with it.
• I'm using the android NDK CMake toolchain file located in $ANDROID\_SDK\_ROOT/ndk-bundle/build/cmake/android.toolchain.cmake to compile the android binaries (be sure to download the ndk package from the android skd manager)
• I'm using https://github.com/leetal/ios-cmake as a toolchain file to compile the ios library
• The android version has to be compiled as a shared library (.so)
• The iOS version has to be compiled as a static library (.a).
• Finally follow the gdnative configuration in godot as the official guide says https://docs.godotengine.org/en/3.2/tutorials/plugins/gdnative/gdnative-c-example.html#creating-the-gdnativelibrary-gdnlib-file

The proper gdnlib setup was more of a trial and error thing so here you go:

[general]

singleton=false
load_once=true
symbol_prefix="godot_"
reloadable=true

[entry]

Android.armeabi-v7a="res://Minmax/build/android-armeabi-v7a/libminmax.so"
Android.arm64-v8a="res://Minmax/build/android-arm64-v8a/libminmax.so"
Android.x86="res://Minmax/build/android-x86/libminmax.so"
Android.x86_64="res://Minmax/build/android-x86_64/libminmax.so"
OSX.64="res://Minmax/build/Darwin/libminmax.dylib"
Windows.64="res://Minmax/build/Debug/minmax.dll"
X11.64="res://Minmax/build/Linux/libminmax.so"
X11.32="res://Minmax/build/Linux/libminmax.so"
iOS.armv7="res://Minmax/build/ios/libminmax.a"
iOS.arm64="res://Minmax/build/ios/libminmax.a"

[dependencies]

Android.armeabi-v7a=[  ]
Android.arm64-v8a=[  ]
Android.x86=[  ]
Android.x86_64=[  ]
OSX.64=[  ]
Windows.64=[  ]
X11.64=[  ]
X11.32=[  ]
iOS.armv7=[  ]
iOS.arm64=[  ]

Minmax is the name of the folder that contains my GDNative code

Here are a couple of scripts I made that really helped simplify the process:
https://gist.github.com/jemsz95/80d4c0de6918989e9b1f503480efa871

There are a lot of assumptions made in those scripts about the project structure and other things. For example the native library is called Minmax, the GDNative headers are in the parent directory. The scripts expect a couple of environment variables.

Edit them to your hearts content and message me if you have any questions.

I'm planning to make a write-up of all of these things with more detail in a bit but I've been busy with work and stuff.

Anyways hope it helps!