Offline, as in blocking every connection? Then you can use a firewall in normal android and block the connections. I did that using RethinkDNS's Firewall.
Ah yes kind of like that, though I said some ROMs specifically (I think it is Graphene OS) because they have the firewall at the system level in the hand of the user, unlike normal Android that needs to use the VPN feature that could potentially experience micro disconnects and if you have Play Services running, then it should have the power to snatch that data from Gboard
Someone was saying something like GMS being still able to exchange data with Gboard so the Firewall most likely isn't really a secure option.
I think that's true. Open source Keyboards are the best choice ig, but I find their UI old or not as good as Gboard, so it's kind of a turn off for me.