Acess GLTF Values

That would create a new material with the same name, but not the same appearance. If you traverse the scene you could perhaps find the material you’re looking for:

const materials = {};
scene.traverse((object) => {
  const material = object.material;
  if (material) {
    materials[material.name] = material;
  }
});

...

mesh.material = materials['Marmore1'];