All the native plugin build projects live in nsgif-projects. The native source (from which all platforms are built) lives in Assets/NSGIF/iOS/.
Nothing special needed - the source gets compiled as part of the normal IL2CPP build process.
On a mac, run nsgif-projects/android/build.sh.
Xcode project in nsgif-projects/mac/. Build artifacts will need to be manually copied to Assets/NSGIF/. Note that the Unity editor does not unload DLLs once loaded and will need restarting to pick up new changes.
VisualStudio project in nsgif-projects/windows/. Build artifacts will need to be manually copied to Assets/NSGIF/x86 and Assets/NSGIF/x86_64. Note that the Unity editor does not unload DLLs once loaded and will need restarting to pick up new changes.