The Canyon Grail with 33-40c tires are ideal for this course, with tread options up to your personal preferences. For tire selection, the tubeless ready gravel offerings from IRC Tire will be a great choice. If stability, durability and cornering grip are your top priority, the IRC Boken 40c is hard to beat.
We would not recommend road bikes for this event. Mountain bikes, while not ideal for road stretches are welcome if you do not own a gravel bike. Tubeless tires and extra tubes are highly recommended.
Low gears will certainly help, as some sections will involve steep terrain. 34-50 chainrings with an 11-32 cassette or 1X equivalents will work great for most participants.