Unity google save cloud service: How to build a good save system?

Hi, I'm having problems to understand what is the best choice to make a good save system that integrates google play save service. I am working on a game from months and i don't know how to start this system, my questions are: Should i make a Json save load system that works even in the case that player is offline? Based on the first question, should i have to make a further Save and Load system by myself that works aside google save cloud service? If so, i should even think how to encrypt json files right? So if the player is online google save cloud service manages the data and if is offline i have another system that manages data anyway, right?