So I have a product built using aws greengrass core and aws iot core. And I'm trying to migrate to Google Cloud Platform. I saw that basically, google cloud iot does the same thing as aws iot core. But I can't really find a google brick with a feature that enable devices to share data when they are not connected to the internet. Is it integrated with cloud iot core ? Or there is no equivalent features on google cloud ?
No, not natively. IoT Core from AWS and GCP have very similar capabilities, but GCP does not have any equivalent to Greengrass. You could implement this manually and still use GCP IoT Core, and leverage Gateways to implement the local bridge to GCP, but you have to do the heavy lifting of how to authenticate devices locally and handle the buffering and message brokering.
You can implement some of that logic and East/West traffic with the LFEdge projects such as EdgeX Foundry or FLEdge, and Open Horizon, but that seems out of scope of your question.