when ever a file is uploaded/overwritten to cloud storage , i wish to download it in real time to local machine
without using gsutil command
You could try using a Cloud Function being triggered by a Create Event in Storage. In that function, you can implement the download side.
See this tutorial. It has a similar objective. Write and deploy an Event-Driven Cloud Function, the function is triggered by uploading a file to Cloud Storage.
Also, see Upload events.