Added documentation for the new GCP storage scaler (#660)

Co-authored-by: Tom Kerkhove <kerkhove.tom@gmail.com>

content/docs/2.7/scalers/gcp-pub-sub.md

@@ -95,7 +95,7 @@ spec:
 
 **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.
+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 intend to use it globally in your cluster.
 
 ### Example using TriggerAuthentication with GCP Identity
 

content/docs/2.7/scalers/gcp-storage.md

@@ -0,0 +1,122 @@
++++
+title = "Google Cloud Platform Storage"
+availability = "2.7+"
+maintainer = "Community"
+description = "Scale applications based on the count of objects in a given Google Cloud Storage (GCS) bucket."
+layout = "scaler"
+go_file = "gcp_storage_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `gcp-storage` scaler, which scales Kubernetes workloads based on the count of objects in a given Google Cloud Storage (GCS) bucket. This scaler assumes the worker, when run, will process and clear the bucket by deleting/moving objects therein.
+
+```yaml
+triggers:
+- type: gcp-storage
+  metadata:
+    bucketName: test-bucket
+    targetObjectCount: '100'
+    maxBucketItemsToScan: '1000'
+    credentialsFromEnv: GOOGLE_APPLICATION_CREDENTIALS_JSON # Optional
+    credentialsFromEnvFile: GOOGLE_APPLICATION_CREDENTIALS_JSON # Optional
+```
+
+**Parameter list:**
+
+- `bucketName` - Name of the bucket in GCS.
+- `targetObjectCount` - Average target value to trigger scaling actions. (Default: `100`, Optional)
+- `maxBucketItemsToScan` - When to stop counting how many objects are in the bucket. (Default: `1000`, Optional)
+As counting the number of objects involves iterating over their metadata it is advised to set this number to the value of `targetObjectCount` * `maxReplicaCount`.
+
+The metric name will be generated automatically based on the trigger index and `bucketName`, for example: **s0-gcp-storage-bucketName**. 
+
+You can provide in the metadata either `credentialsFromEnv` or `credentialsFromEnvFile`.
+- `credentialsFromEnv` - Set to the name of the environment variable that holds the credential information.
+- `credentialsFromEnvFile` - Set to the name of a json file that holds the credential information.
+
+### Authentication Parameters
+You can use `TriggerAuthentication` CRD to configure the authenticate by providing the service account credentials in JSON. 
+
+**Credential based authentication:**
+
+- `GoogleApplicationCredentials` - Service account credentials in JSON.
+
+**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 intend to use it globally in your cluster.
+
+### Examples
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: gcp-storage-scaledobject
+  namespace: keda-gcp-storage-test
+spec:
+  scaleTargetRef:
+    name: keda-gcp-storage-go
+  triggers:
+  - type: gcp-storage
+    metadata:
+      bucketName: "Transactions"
+      targetObjectCount: "5"
+      credentialsFromEnv: GOOGLE_APPLICATION_CREDENTIALS_JSON
+```
+
+#### Use TriggerAuthentication with Kubernetes secret
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: keda-trigger-auth-gcp-credentials
+spec:
+  secretTargetRef:
+  - parameter: GoogleApplicationCredentials 
+    name: gcp-storage-secret        # Required. Refers to the name of the secret
+    key: GOOGLE_APPLICATION_CREDENTIALS_JSON       # Required.
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: gcp-storage-scaledobject
+spec:
+  scaleTargetRef:
+    name: keda-gcp-storage-go
+  triggers:
+  - type: gcp-storage
+    authenticationRef:
+      name: keda-trigger-auth-gcp-credentials
+    metadata:
+      bucketName: "Transactions"
+      targetObjectCount: "5"
+```
+
+#### Use 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: gcp-storage-scaledobject
+spec:
+  scaleTargetRef:
+    name: keda-gcp-storage-go
+  triggers:
+  - type: gcp-storage
+    authenticationRef:
+      name: keda-trigger-auth-gcp-credentials
+    metadata:
+      bucketName: "Transactions"
+      targetObjectCount: "5"
+```