It's not only about the actual words you're typing. This is one thing and you can opt to "trust" Google that they do what they say (which is not always the case, as previous scandals show). But it's also about other data the keyboard collects, such as contact details, memory data, usage statistics, etc. If you're comfortable with sharing everything with Google, well then that's that. I'd then still ask if Gboard is 100% comfortable to use, whether you never do a typo, whether you feel there's no improvement possible :-)