How should I train for gravel rallies?
I suggest given your goals (finish and enjoy) you can train for fitness largely on road, but try to get out and train on gravel whenever you can.  

I've had some fun on my tourer on gravel tracks recently and can say that it's well worth getting in some practice on that surface, in a range of conditions -- the bike will handle very differently.  You should choose your gravel tyres soon and use them for practice (there's no real harm in leaving them on for on-road training in your case).

Luckily your goals are compatible with not having much chance to ride the course in advance, as it looks like you're planning to travel to at least one of the events. Also the Dirty Boar doesn't permit riding in advance as they've got special permission for some of the route.

In your position I could be quite tempted to sign up for the 130km version and just go for it with only enough specific training to try out your gravel tyres (and bike?) on gravel.