Its kinda is possible (on some android setups) but unreliable.
Most importantly your app (and server) needs to be build to be able to fall back to a 3rd party message broker. But the common fallback is to just sync messages from time to time in background if the app runs as it's "good enough" for the case Google is temporary down or not available or you are one of the (from the App POV) view people which de-googled their phone.
Most importantly your app (and server) needs to be build to be able to fall back to a 3rd party message broker. But the common fallback is to just sync messages from time to time in background if the app runs as it's "good enough" for the case Google is temporary down or not available or you are one of the (from the App POV) view people which de-googled their phone.