add GCP to heroku migration docs

Author: jordanstephens

docs/tutorials/migrating-from-heroku.md

@@ -13,7 +13,7 @@ This tutorial will guide you through the process of migrating your Heroku applic
 * [The Heroku CLI (optional, but recommended)](https://devcenter.heroku.com/articles/heroku-cli#install-the-heroku-cli)
 * Cloud Account Credentials
   * [AWS](https://docs.aws.amazon.com/cli/latest/userguide/cli-chap-authentication.html)
-  * [GCP](https://cloud.google.com/docs/authentication/set-up-adc-local-dev-environment) (coming soon)
+  * [GCP](https://cloud.google.com/docs/authentication/set-up-adc-local-dev-environment)
 
 :::tip
 **Do I need a Dockerfile?**
@@ -167,7 +167,7 @@ Now all you need to do is deploy your application to the cloud.
 
 ### Deploying to AWS
 
-If you're deploying to AWS, you'll need to invoke `defang compose up` with your AWS access credentials in the environment:
+If you're deploying to AWS, you'll need to invoke `defang compose up --provider aws` with your AWS access credentials in the environment:
 
 <details>
 <summary>
@@ -285,7 +285,68 @@ See our full tutorial on [deploying to AWS](/docs/tutorials/deploy-to-aws).
 
 ### Deploying to GCP
 
-GCP support for deployments without Dockerfiles coming soon.
+If you're deploying to GCP, you'll need to invoke `defang compose up --provider gcp` with your GCP access credentials in the environment:
+
+<details>
+<summary>
+Run the following command
+```
+GCP_PROJECT_ID=my-project-123456 defang compose up --provider gcp
+```
+</summary>
+```
+ * Using Google Cloud Platform provider from command line flag
+ ! Defang cannot monitor status of the following managed service(s): [postgres release].
+   To check if the managed service is up, check the status of the service which depends on it.
+ ! service "postgres": missing memory reservation; using provider-specific defaults. Specify deploy.resources.reservations.memory to avoid out-of-memory errors
+ ! service "web": ingress port without healthcheck defaults to GET / HTTP/1.1
+ * Packaging the project files for web at /Users/defang/wk/vast-badlands
+ * Uploading the project files for web
+ * Setting up defang CD in GCP project my-project-123456, this could take a few minutes
+ * Packaging the project files for release at /Users/defang/wk/vast-badlands
+ * Uploading the project files for release
+ * Tailing logs for deployment ID fgdh4ct0hopz ; press Ctrl+C to detach:
+ * Showing only build logs and runtime errors. Press V to toggle verbose mode.
+2025-09-08T10:28:08.634-04:00 cd  ** Update started for stack beta
+2025-09-08T10:28:17.990-04:00 cd  ** Updating service "postgres"
+2025-09-08T10:28:44.773-04:00 cd  ** Building image for "release"...
+2025-09-08T10:28:58.012-04:00 cd  ** Building image for "web"...
+2025-09-08T10:47:02.479-04:00 release I, [2025-09-08T14:47:02.409371 #1]  INFO -- : Migrating to CreateMembers (20240416182733)
+2025-09-08T10:47:02.479-04:00 release == 20240416182733 CreateMembers: migrating ====================================
+2025-09-08T10:47:02.479-04:00 release -- create_table(:members)
+2025-09-08T10:47:02.479-04:00 release    -> 0.0073s
+2025-09-08T10:47:02.479-04:00 release == 20240416182733 CreateMembers: migrated (0.0073s) ===========================
+2025-09-08T10:47:02.479-04:00 release I, [2025-09-08T14:47:02.432075 #1]  INFO -- : Migrating to DeviseCreateUsers (20240417165503)
+2025-09-08T10:47:02.479-04:00 release == 20240417165503 DeviseCreateUsers: migrating ================================
+2025-09-08T10:47:02.479-04:00 release -- create_table(:users)
+2025-09-08T10:47:02.479-04:00 release    -> 0.0080s
+2025-09-08T10:47:02.479-04:00 release -- add_index(:users, :email, {:unique=>true})
+2025-09-08T10:47:02.479-04:00 release    -> 0.0022s
+2025-09-08T10:47:02.479-04:00 release -- add_index(:users, :reset_password_token, {:unique=>true})
+2025-09-08T10:47:02.479-04:00 release    -> 0.0021s
+2025-09-08T10:47:02.479-04:00 release == 20240417165503 DeviseCreateUsers: migrated (0.0124s) =======================
+2025-09-08T10:47:02.479-04:00 release I, [2025-09-08T14:47:02.458370 #1]  INFO -- : Migrating to AddUserIdToMembers (20240417202202)
+2025-09-08T10:47:02.479-04:00 release == 20240417202202 AddUserIdToMembers: migrating ===============================
+2025-09-08T10:47:02.479-04:00 release -- add_column(:members, :user_id, :integer)
+2025-09-08T10:47:02.479-04:00 release    -> 0.0044s
+2025-09-08T10:47:02.479-04:00 release -- add_index(:members, :user_id)
+2025-09-08T10:47:02.479-04:00 release    -> 0.0019s
+2025-09-08T10:47:02.479-04:00 release == 20240417202202 AddUserIdToMembers: migrated (0.0064s) ======================
+2025-09-08T10:48:16.157-04:00 web => Booting Puma
+2025-09-08T10:48:16.157-04:00 web => Rails 7.1.3.2 application starting in production
+2025-09-08T10:48:16.157-04:00 web => Run `bin/rails server --help` for more startup options
+2025-09-08T10:48:18.625-04:00 web [1] Puma starting in cluster mode...
+2025-09-08T10:48:18.625-04:00 web [1] * Puma version: 6.4.2 (ruby 3.3.4-p94) ("The Eagle of Durango")
+2025-09-08T10:48:18.625-04:00 web [1] *  Min threads: 5
+2025-09-08T10:48:18.625-04:00 web [1] *  Max threads: 5
+2025-09-08T10:48:18.625-04:00 web [1] *  Environment: production
+2025-09-08T10:48:18.626-04:00 web [1] *   Master PID: 1
+2025-09-08T10:48:18.626-04:00 web [1] *      Workers: 2
+2025-09-08T10:48:18.626-04:00 web [1] *     Restarts: (✔) hot (✔) phased
+2025-09-08T10:48:18.626-04:00 web [1] * Listening on http://0.0.0.0:5000
+2025-09-08T10:48:18.627-04:00 web [1] Use Ctrl-C to stop
+```
+</details>
 
 ## Step 3 - Migrating your data