Random Kubernetes/GKE question here. If I were to update an ingress object configured to use the cloud providers ingress controller, but was updating it to switch to an nginx-ingress controller, would the finalizer be invoked, which cleans up external LoadBalancers? Or would it effectively orphan those external LBs?

Finalizers are only invoked when objects are deleted, not when they are changed. Whether or not its handled properly depends on how the cloud providers lb controller is coded.