And here's exactly the challenge. Google will not tell you how exactly they use your data. What you can check though is what data the keyboard is able to access, and then imagine the worst. Here's an excerpt from Play store, of Gboard's permissions:

find accounts on the device read your contacts view Wi-Fi connections read your own contact card find accounts on the device read the contents of your USB storage take pictures and videos record audio download files without notification full network access control vibration read terms you added to the dictionary measure app storage space use accounts on the device prevent device from sleeping read Google service configuration run at startup view network connections add words to user-defined dictionary