Alpha feature testing is invite-only. Beta feature testing is publicly available. And once a feature is classified as General Availability (GA), it will no longer be available for alpha/beta. See status and availability distinction
here.
One of the best places to get the latest news on GCP products is by visiting the
Google Cloud Platform blog or signing up to the
GCP Newsletter. You can also find new features on the docs. For example on
this page, GAE standard features are labeled beta, alpha, third-party, etc.