fix: Fix publish from EG guide (#870)

Author: alexbouchardd

docs/content/publishing/publish-from-hookdeck-event-gateway.mdoc

@@ -1,77 +1,18 @@
 ---
-title: "Publish from GCP Pub/Sub"
-description: "Configure Outpost to ingest published events from a GCP Pub/Sub topic and subscription."
+title: "Publish from Hookdeck Event Gateway"
+description: "Configure Outpost to ingest published events from Hookdeck Event Gateway."
 ---
 
-This guide provides information on using GCP Pub/Sub to publish events to Outpost.
+This guide provides information on using Hookdeck Event Gateway to publish events to Outpost.
 
-## Message Structure
+## Setup the Outpost API as a Destination
 
-GCP Pub/Sub messages should have the same payload structure as the [Publish API endpoint](/docs/outpost/publishing/events).
+To publish to Outpost via the Event Gateway, you can set your HTTP destination to point to the Outpost Publish API endpoint ({% $OUTPOST_API_BASE_URL %}/publish).
 
-```json
-{
-  "tenant_id": "<TENANT_ID>",
-  "destination_id": "<DESTINATION_ID>", // Optional. Provide a way of routing events to a specific destination
-  "topic": "topic.name", // Topic defined in TOPICS environment variable
-  "eligible_for_retry": true | false, // Should event delivery be retried? Default is true.
-  "metadata": Payload, // can be any JSON payload,
-  "data": Payload // can be any JSON payload
-}
-```
+Enable Bearer Authentication with your Outpost API key.
 
-## GCP Pub/Sub Setup
+## Payload shape & transformation
 
-[Create a topic](https://cloud.google.com/pubsub/docs/create-topic) and [create a pull subscription](https://cloud.google.com/pubsub/docs/create-subscription) within GCP Pub/Sub.
+The payload shape must match the expected shape of the [Publish API endpoint](/docs/outpost/publishing/events).
 
-Once you have these in place, you can configure Outpost to subscribe to receive events and then publish them to subscribed Outpost Destinations.
-
-## Configuration
-
-Provide Outpost with credentials, topic, and subscription for your GCP Pub/Sub instance used for publishing events.
-
-### Environment Variables
-
-```
-PUBLISH_GCP_PUBSUB_PROJECT=""
-PUBLISH_GCP_PUBSUB_SERVICE_ACCOUNT_CREDENTIALS=""
-PUBLISH_GCP_PUBSUB_TOPIC=""
-PUBLISH_GCP_PUBSUB_SUBSCRIPTION=""
-```
-
-#### Example
-
-```
-PUBLISH_GCP_PUBSUB_PROJECT="my-gcp-project"
-PUBLISH_GCP_PUBSUB_SERVICE_ACCOUNT_CREDENTIALS="single_line_json_string"
-PUBLISH_GCP_PUBSUB_TOPIC="outpost-publish"
-PUBLISH_GCP_PUBSUB_SUBSCRIPTION="outpost-publish-sub"
-```
-
-### YAML
-
-```yaml
-publishmq:
-  gcp_pubsub:
-    project: <GCP_PROJECT>
-    service_account_credentials: <GCP_SERVICE_ACCOUNT_CREDENTIALS>
-    topic: <GCP_PUBSUB_TOPIC>
-    subscription: <GCP_PUBSUB_SUBSCRIPTION>
-```
-
-#### Example
-
-```yaml
-publishmq:
-  gcp_pubsub:
-    project: "GCP_PUBSUB_PROJECT" # GCP project ID
-    service_account_credentials: "GCP_PUBSUB_SERVICE_ACCOUNT_CREDENTIALS" # Contents of service account credentials JSON file
-    topic: "outpost-publish" # Pub/Sub to read for published events
-    subscription: "outpost-publish-sub" # Pub/Sub subscription for published events
-```
-
-### Troubleshooting
-
-- [Ask a question](https://github.com/hookdeck/outpost/discussions/new?category=q-a)
-- [Report a bug](https://github.com/hookdeck/outpost/issues/new?assignees=&labels=bug&projects=&template=bug_report.md&title=%F0%9F%90%9B+Bug+Report%3A+)
-- [Request a feature](https://github.com/hookdeck/outpost/issues/new?assignees=&labels=enhancement&projects=&template=feature_request.md&title=%F0%9F%9A%80+Feature%3A+)
+You can use a [transformation](/docs/transformations) to modify the payload shape before it is published to Outpost.