docs: add gcp engine auth fields to frontmatter reference (#49120)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

docs/src/content/docs/reference/frontmatter-full.md

@@ -2455,8 +2455,9 @@ engine:
     # (optional)
     azure-cloud: "example-value"
 
-    # Optional WIF provider discriminator. Recognized values are 'azure' and
-    # 'anthropic'.
+    # Optional WIF provider discriminator. Recognized values are 'azure',
+    # 'anthropic', and 'gcp'. Use 'gcp' for Gemini Vertex AI authentication via
+    # Google Cloud Workload Identity Federation.
     # (optional)
     provider: "example-value"
 
@@ -2476,6 +2477,22 @@ engine:
     # (optional)
     workspace-id: "example-value"
 
+    # Google Cloud Workload Identity Provider resource name for Vertex AI WIF.
+    # (optional)
+    workload-identity-provider: "example-value"
+
+    # Google Cloud service account email to impersonate for Vertex AI WIF.
+    # (optional)
+    service-account: "example-value"
+
+    # Google Cloud project ID for Vertex AI WIF.
+    # (optional)
+    project: "example-value"
+
+    # Google Cloud location for Vertex AI WIF (defaults to us-central1).
+    # (optional)
+    location: "example-value"
+
   # Additional TOML configuration text that will be appended to the generated
   # config.toml in the action (codex engine only)
   # (optional)