If you don't trust Google, why use Google?
Discussion
Because I've eliminated all other Google services from my life. Having an offline keyboard made by Google isn't that big a deal for me.
It's a keylogger if you keep it online though. And eventually there'll be an open source keyboard that can stand up to Gboard.
I wouldn't trust their "offline" setting. There's no match for Google Maps, and I still refuse to use it. I'd prefer less features if it means not giving them any data, including "number of installs/hits".