How can I get datalayer path?

How can I get the datalayer path (i.e. datalayer._url) when the user clicks an gltf building model?

Are you trying to obtain the url that the glTF was loaded from? I would use scene.pick() to get the entity that was clicked on, and then you can save the URL (or whatever other custom properties) on this object just like you would with any regular JavaScript object.