After reading the docs
I think they are mostly the same.
I can use GAE cron job + pub/sub + cloud function to implement the same functions which cloud scheduler has.
In my understanding, it seems there are some differences between them:
Cloud Scheduler can be more convenient to adjust frequency. To update the frequency of GAE cron job, you must update the config, like schedule: every 1 hours of cron.yaml and redeploy.
There is no need to implement the cron job architecture(integrate GAE, GAE cron service, pub/sub, cloud function, etc..) by yourself which means you don't need to write code for combining them together anymore.
Am I correct? Or, any other differences?
You're right in that the Google Cloud Scheduler is kind of an evolution of the GAE cron job mechanism to make it more user-friendly and flexible. You can see that they are still related since the Cloud Scheduler doc specifies:
To use Cloud Scheduler your project must contain an App Engine app that is located in one of the supported regions. If your project does not have an App Engine app, you must create one.
Historically, GAE cron job was the only cron service offered by the platform. You could only target a GAE handler to receive the request from cron. From there you could indeed perform actions like like publish on pub/sub, call an HTTP Cloud Function or launch a dataflow job, but you always had to deploy a GAE service to handle it, which wasn't optimal.
The new Cloud Scheduler makes it more straightforward to use with Pub/Sub, Cloud Functions but also any publicly available HTTP endpoint (may be on-premise). And of course App Engine handlers. More targets may be added in the future for more use-cases.
Finally, as you mentioned, the API exposed to manage it decouples it from App Engine and its cron.yaml file and makes it easier to create and update cron jobs dynamically.