I rode it in 2018 on a gravel bike (on 47mm WTB Horizons), which felt like a good compromise, and I'd probably choose a gravel bike again. There's a lot of road miles and a LOT of climbing. Any smaller than a 40c tyre would have been tough going on some of the rougher gravel sectors. I'm sure a mtb with fast rolling tyres would get you round absolutely fine too though.
I rode it in 2018 on a gravel bike (on 47mm WTB Horizons), which felt like a good compromise, and I'd probably choose a gravel bike again. There's a lot of road miles and a LOT of climbing. Any smaller than a 40c tyre would have been tough going on some of the rougher gravel sectors. I'm sure a mtb with fast rolling tyres would get you round absolutely fine too though.