Add new gcp identity scaler option in docs (kedacore#565)

Signed-off-by: Jose Maria Alvarez <jose-maria.alvarez@leroymerlin.es>

content/docs/2.5/scalers/gcp-pub-sub.md

@@ -82,3 +82,33 @@ spec:
     metadata:
       subscriptionName: "input" # Required  
 ```
+
+**Identity based authentication:**
+
+You can also use `TriggerAuthentication` CRD to configure the authentication using the associated service account of the running machine in Google Cloud. You only need to create a `TriggerAuthentication` as this example, and reference it in the `ScaledObject`. `ClusterTriggerAuthentication` can also be used if you pretend to use it globally in your cluster.
+
+### Example using TriggerAuthentication with GCP Identity
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: keda-trigger-auth-gcp-credentials
+spec:
+  podIdentity:
+    provider: gcp
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: pubsub-scaledobject
+spec:
+  scaleTargetRef:
+    name: keda-pubsub-go
+  triggers:
+  - type: gcp-pubsub
+    authenticationRef:
+      name: keda-trigger-auth-gcp-credentials
+    metadata:
+      subscriptionName: "input" # Required
+```