You might be affected with the recent changes. Try what I mentioned here. Specifically, try generating a Server Key again, but this time, through the Firebase Console, then try a request again. If you're requests are still failing, and if you can see an error from the GCM Server, I suggest contacting GCM Support.
If you are just starting to use GCM, instead of creating a project in the Google Developers Console, do it in the Firebase Console. After creating the project, simply use the auto-generated Server Key. Here are the steps where to find the Server Key:
- Go to your Firebase Console and click on CREATE NEW PROJECT.
- Fill in your desired Project Name and select your Country. After this, the new Project should be active.
- Then on left-side panel, click on the gear button and select Project Settings.
- Then go to the Cloud-Messaging Tab.
For old GCM projects, you can simply Import the project to the Firebase Console:
- Go to your Firebase Console and click on IMPORT PROJECT.
- Select the project you want to import and your country.
- Click on ADD FIREBASE. After this, the new Project should be active.
- Then on left-side panel, click on the gear button and select Project Settings.
- Then go to the Cloud-Messaging Tab.