How to use your own domain in Google Cloud Platform

Since the tag is GAE, I will answer on the assumption that it is GAE.The GCE is the same as a regular server, so you can assign DNS to static IPs.
In the case of a GAE, if you direct CNAME to google with the name ghs.googlhosted.com and specify the domain from the administrative console of the GAE project, you will be directed to that service of the GAE.

Detailed instructions are available on the official website, so please refer to the following.
https://cloud.google.com/appengine/docs/python/console/using-custom-domains-and-ssl

Previously, Google Apps was a must, but since about a year or two ago, domain settings have become possible only with GCP, and SSL has been completed with GCP since this year.
That's why old commentary articles in Japanese are useless, so this is the only one I have right now.