You can create a custom domain (including naked domain) for your App Engine and you can also set up self managed SSL certificates there.
To use a custom domain, map the domain to your app, then update your DNS records. You can map a naked domain, such as example.com or a subdomain, such as subdomain.example.com. You can also use wildcards to map subdomains.
The steps to map custom domain to your application are following -
In Console,go to the Application settings tab of the App Engine Settings page
-->If you need to enable G Suite authentication then click Edit to modify the Google Accounts API Referrer-->In the Google Authentication drop-down menu, select G Suite domain, then add your domain such as example.com in the empty field.
Go to the Custom Domains tab of the App Engine Settings page--> Click on add custom Domain-->If your Domain is already verified then select it from the drop down menu-->click Continue.
If you haven't verified your domain yet, follow the steps below:
a) Select Verify a new domain from the drop-down menu.
b) Enter your naked domain name (such as "example.com") and click Verify.
c) Enter information in the Webmaster Central window that appears.
d) After you complete the steps in Webmaster Central, return to the Add a new
custom domain page in the Google Cloud Console.
In the Point your domain to (project-ID) section, specify the domain and subdomains that you want to map.We recommend mapping the naked domain and the www subdomain-->click Save mapping.
Sign in to your domain registrar web site and update your DNS records.
Please refer to the following link for detailed description of required steps -
https://cloud.google.com/appengine/docs/standard/python/mapping-custom-domains