WhoTheFuckisAlice The GrapheneOS keyboard is based on the standard AOSP keyboard, which is open source. You can verify the source code for whatever keyboard, or any software for that matter, that you use.
The default keyboard has no permissions by default except for the optional Contacts permission for contact suggestions, and even that can be heavily enforced with Contact Scopes. Since the keyboard has no Network permissions either, it won't send any data to the Internet.