Same issue I was facing and able to solve !
Background : In my scenario we have an iOS App in the store and all our push notification configured through Google GCM (We were not using FCM). But we had situation where our APNS certificate is just about to get expired and we don't want to make any iOS Release(We had around 4k DAU on the current version)
Once certificate was expired any Notification we sent was throwing InvalidApnsCredential
from GCM.
Here is how to get out of this.
- Configure you Firebase project. (Make sure you're logged into right Google account - the one you configured for setting up your GCM)
Goto : https://firebase.google.com > GET STARTED
Choose Import Google Project
You must see now all the GCM Project as below - If your logged in with correct account.
Choose the project.
Now your Firebase project is ready. It should look like below.
To Update new certificate.
- Choose Setting gear at Overview.
- Choose Project Settings
Now choose Cloud Messaging Tab & now you APNs Certificate section is visible.
- If Certificate already expired - you must see RED flagged cell with UPLOAD & DELETE options. Upload new APNs certificate using * UPLOAD*.

You got better way to do it, please share your thoughts ! Cheers !