Google公式ドキュメント
https://developers.google.com/cloud-messaging/faq

Googleの公式ドキュメントを見る限り、共存は可能です。

Will my existing GCM tokens and token handling still work?
Your GCM and FCM registration tokens are mutually interchangeable; you can send to existing GCM tokens through the FCM HTTP server APIs. For GCM and FCM token handling, remember:
– If you store and send to specific tokens, be sure to implement the token refresh method to ensure your tokens are always up to date
– iOS devs should avoid calling .getToken(authorizedEntity,scope) unless there is a need to enable multiple senders. Use FirebaseInstanceId.getInstance().getToken() instead.

ただ、2019年4月11日にGCMはサポートを終了するため、それまでにはFCMに移行する必要があります。

As of April 10, 2018, Google has deprecated GCM. The GCM server and client APIs are deprecated and will be removed as soon as April 11, 2019. Migrate GCM apps to Firebase Cloud Messaging (FCM), which inherits the reliable and scalable GCM infrastructure, plus many new features.