Error from server (Forbidden): error when retrieving current configuration of: Resource: "apps/v1, Resource=deployments",
Going from this, your ClusterRole isn't configured to allow access to deployments, and the ClusterRole you've listed isn't properly bound to your service account. You could configure it with something like I did below as a troubleshooting measure/to make sure you're able to properly configure permissions and rule out an issue with the serviceaccount's role bindings. 

apiVersion: v1
kind: ServiceAccount
metadata:
  name: gitlab-admin
  namespace: kube-system
---
apiVersion: rbac.authorization.k8s.io/v1beta1
kind: ClusterRoleBinding
metadata:
  name: gitlab-admin-binding
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: deployment-reader
subjects:
- kind: ServiceAccount
  name: gitlab-admin
  namespace: kube-system


A new ClusterRole

kind: ClusterRole
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: deployment-reader
rules:
- apiGroups: ["extensions", "apps"]
  resources: ["deployments"]
  verbs: ["get", "watch", "list"]


Then to check to make sure the service account can properly access the resource in the default namespace you can check with the following command

kubectl get deployments --as system:serviceaccount:kube-system:gitlab-admin -n default