Google can shuttle network requests to any of their other apps (or play services, if that's running) via IPC, even with the network permission disabled in Gboard.

So if you're using Graphene for privacy from Google specifically, it's not safe to install things like Gboard even with the network permission disabled. Even if they don't use IPC in this way today, there's no telling what the Google-of-tomorrow will do.

The latest FUTO works just as well as Gboard for me. Their somewhat recent update to their swiping library really changed the game, and their voice recognition is also pretty decent.

fyi disabling network permission doesn't protect against deliberate and coordinated data exfiltration. Apps can still communicate with other apps, for example with Google Play Services or any other app.