The majority of the Grom Pre De Walcha route is on well managed public dirt roads. The optimal bike for the Grom Pre is somewhere between a gravel bike with 40mm+ tires and a mountain bike with 2.2” tires.
This route is certainly do-able on less and more if you don't have a bike within that range but know your ability.
There is a break down of what is on my shit hot Crust Bombora here. This would be a good style of bike to ride.