[ECF GCP] Add documentation for tech preview

Author: constanca-m

docs/reference/edot-cloud-forwarder/gcp.md

@@ -0,0 +1,109 @@
+---
+navigation_title: GCP
+description: Set up the EDOT Cloud Forwarder for GCP to bring your GCP logs to Elastic Observability.
+applies_to:
+  serverless:
+    observability: preview
+  deployment:
+    ess: preview
+  product:
+    edot_cf_gcp: preview
+products:
+  - id: cloud-serverless
+  - id: observability
+  - id: edot-cf
+---
+
+# EDOT Cloud Forwarder for GCP
+
+{{edot-cf}} for GCP provides a serverless, scalable way to ingest Google Cloud Platform logs into Elastic. It deploys the EDOT Collector as a Google Cloud Run service that listens for Pub/Sub push subscriptions, processes the logs, and forwards them to {{motlp}}.
+
+## Architecture overview
+
+The architecture for the {{edot-cf}} GCP is as pictured:
+
+![EDOT Cloud Forwarder GCP overview](../images/edot-cloud-forwarder-gcp-overview.svg)
+
+### Data flow
+
+- Ingestion: Logs are sent to a Pub/Sub topic (either directly or via GCS Bucket notifications).
+- Processing: A push subscription triggers the Cloud Run service, where the {{edot-cf}} is running.
+- Forwarding: The service processes the data and exports it to Elastic via the {{motlp}}.
+- Failure Handling: If processing fails (after retries), messages are routed to a dead letter topic and archived in a GCS Bucket for future analysis.
+
+## Supported log types
+
+Currently, {{edot-cf}} for GCP supports the following log types:
+
+% TODO constanca-m MAYBE add permalink to otel version we will use in ECF GCP image in the table
+
+| Log             | OTel mapping    |
+|-----------------|-----------------|
+| Cloud Audit Log | Cloud Audit Log |
+| VPC Flow Log    | Access logs     |
+
+:::{note}
+We are working to support other popular log types and sources. [Contact us](docs-content://troubleshoot/ingest/opentelemetry/contact-support.md) to let us know of any specific requirements that could influence our plans.
+:::
+
+
+## Prerequisites
+
+### Elastic requirements
+
+- Access to {{motlp}} endpoint.
+- Valid API key with ingest permissions.
+
+You can refer to [Send data to Elastic](../motlp.md#send-data-to-elastic) documentation for more details.
+
+
+### GCP permissions
+
+% TODO constanca-m joecompute https://github.com/elastic/edot-cloud-forwarder-gcp/pull/225
+
+
+## Quick start
+
+% TODO Publish https://github.com/elastic/terraform-google-edot-cloud-forwarder on terraform public registry
+
+
+## Features
+
+The {{edot-cf}} is designed for reliability and observability.
+
+### Flexible ingestion
+
+Logs can be sent:
+
+- Directly to a Pub/Sub topic.
+- To a file placed in a GCS bucket. This will trigger an event notification to Pub/Sub which in turn will trigger the {{edot-cf}}.
+
+### Reliability & Recovery
+
+- Dead letter queue (DLQ): If a log entry fails to process or send to Elastic after the configured retries, it is not lost. The
+  {{edot-cf}} automatically routes failed messages to a dedicated GCS bucket for later analysis.
+- Smart retries: Built-in exponential backoff for transient network issues.
+
+### Observability & Metadata
+
+- Self-telemetry: You can configure the collector to send its own internal telemetry to the {{motlp}}.
+- Enrich metadata: You can enable `include_metadata` to enrich your logs with context from the transport layer, including:
+  - `bucket`
+  - `object`
+  - `subscription`
+  - `message_id`
+  - `delivery_attempt`
+
+## Performance
+
+% TODO
+
+## Limitations
+
+% TODO Add information on: permanent errors don't stop the retries
+
+## Changelog
+
+% How to link the CHANGELOG.md file if it is in a private repository?
+% https://github.com/elastic/edot-cloud-forwarder-gcp/blob/main/CHANGELOG.md
+