Hello Sankar,
A local emulator for Google Cloud Storage is not currently offered for offline usage. I have found a similar issue on github where a user created an emulated (mock) bucket for his javascript application, and shared it with the community. You may check it out and programmatically create a similar solution for your use case. On the other hand, and in order to have the product team look into it, I encourage you to submit a feature request here. I hope this helps.