I don't use Google Sheets very often, so maybe this is obvious to someone that does. But, how do you look at the data structure and organization underneath Google Sheets? Are you an employee or is this available to the general public? Through an api?
In the case of open source software, a specific open source app or library or API, often all it takes is to reference a library or API by name, use a few terms specific to that library or service, and you'll get an LLM context that retrieves/generates from that area of knowledge. That context knows information about the library/API that is useful for integration and will tell you all about them just by explaining what you're trying to do. Note this is essentially conversational R&D, which is then backed up by verifying the data structures, library calls and/or APIs are indeed as they are being discussed. I include verifying checks during the conversation, rather than build a strategy and then check if what was discussed is even possible.
If an integration is possible, if the above conversation yields useful data and function/API calls (which they tend to do), then it's just a matter of a well structured prompt that's probably generating structured output. The end result is a process where a useful library/API call retrieves essential data of some sort that the user can ask questions about, modify, and if modified the prompt generates the data that is then fed back into the library/API.