L8437
Gboard with networking on and Google Services installed/enabled, can and does occasionally send words you type. They do this to built a "better dictionary" in your language, to learn new terms, phrases, slang, etc. If you're also logged into Google, they also build a profile of your personal dictionary in order to do better word prediction (even if your not using prediction).
Full keylogs going to the cloud probably isn't practical for Google though. They can build a unique profile of the words you use mostly locally, and send the metadata. In browser incognito modes, Gboard also goes into an "incognito" mode where they say they aren't using any key logging at all.
With networking off, I don't see Gboard sending anything.
I've been using Gboard with network blocked and offline speech to text downloaded and working.