Godot uses source generators to fill out C# partial classes, so it doesn't quite have native F# support. You'd at least need C# shims for your types that inherit from Godot classes.
Bummer, I had no idea about that. But I guess that, at least for turn-based games, it wouldn't be super annoying if you manage to split the logic (F#) from the UI glue (C#), right?
(I never got around to that point though, I usually start making games as libraries and then get nerd sniped into reading about game theory for the AI before I make them playable for humans.)