To enable SSL for a custom domain in App Engine, you need to map your custom domain to your App Engine application and then configure an SSL certificate for it. Here are the steps to do this:

1. Verify your custom domain:
   a. Go to the GCP Console: https://console.cloud.google.com/
   b. Select your project.
   c. Navigate to App Engine by clicking the hamburger menu and then click on “App Engine.”
   d. Click on “Settings” in the left side menu.
   e. Click on “Custom domains” tab.
   f. Click on “Add a custom domain.”
   g. Follow the prompts to verify your domain ownership using the provided DNS records.
2. Map your custom domain to your App Engine application:
   a. After verifying your domain, you’ll be prompted to map it to your App Engine app.
   b. Choose a subdomain (e.g., ‘www’) or the root domain (e.g., ‘example.com’) and click “Continue.”
   c. Update your domain’s DNS records with the provided records to complete the mapping process.
3. Configure an SSL certificate for your custom domain:
   a. Once the domain mapping is complete, navigate back to the “Custom domains” tab under App Engine settings.
   b. Google automatically provisions and manages free SSL certificates for your mapped custom domains using Let’s Encrypt. It may take some time for the SSL certificate to be provisioned and become active.
   c. Once the SSL certificate is active, it will show “Managed certificate” under the “SSL security” column for your custom domain.
   d. If you prefer to use your own SSL certificate, click on the domain, and then click on “Upload a new SSL certificate.” Follow the prompts to upload your custom SSL certificate and private key.

After completing these steps, your custom domain will have SSL enabled, and your App Engine application will be served over HTTPS.