The documentation for the GCM API is regrettably out of date. When this API was first introduced it was powered by Google Cloud Messaging, but in 2019 (I think) it we moved it over to use Firebase Cloud Messaging. There are no plans to remove this capability from the extensions platform.
On a related note, I opened
issue 1337382 to suggest that we rename the API from
chrome.gcm to something less confusing. In order to avoid breaking existing extensions, I expect that we won't take action on this request until the next manifest version bump.
I also opened
issue 1319986 to request that Chromium not require that extension service workers display a notification when they receive messages with the web's
push API.
Simeon - @dotproto
Chrome Extensions DevRel