Import

var path; // gltf, glb or zip(include gltf)

var context = gltfImporter.Load(path);
context.ShowMeshes();

GameObject root = context.Root;

Export

GameObject go; // input GameObject
String path; // output file path

var gltf = new glTF();
using (var exporter = new gltfExporter(gltf))
{
    exporter.Prepare(go);
    exporter.Export();
}
var bytes = gltf.ToGlbBytes();
File.WriteAllBytes(path, bytes);