Hey man I’m struggle-bussing my way through some GDExtension stuff too - this thread here has lots of good info, maybe it would help you
https://forum.godotengine.org/t/returning-custom-classes-from-c-gdextension/65920
also the Terrain3D project is up on github and a great example of a GDExtension that has a resource it defines that uses an array of images (or textures, anyways). Maybe check that out too.