I want to fetch FCM tokens using a gradle task; however google sort of forces you to use their dependency

FileInputStream serviceAccount =
new FileInputStream("path/to/serviceAccountKey.json");

FirebaseOptions options = new FirebaseOptions.Builder()
  .setCredentials(GoogleCredentials.fromStream(serviceAccount))
  .build();

FirebaseApp.initializeApp(options);

Which is a java dependency available via maven Can I somehow include a 3rd party dependency just for that one task?

Sure, use workerexecutor with a custom classpath.