In first gear on loose gravel, I have had wheel spin problems with my Nomad Mk II, both unladen and with a heavy load. When your gear ratio is low enough and your hill is steep enough, loose gravel can be a problem because that low gear gives you a lot of wheel torque.
First photo, if you look down, you can see some switchback gravel road, I had a lot of trouble with wheel spin there. But, I needed that low enough of a gear to climb the hill. Each time it spun and I lost momentum, I would have to push the bike to the next turn where it would be flat enough for me to start out pedaling on the bike again. I had a 57mm wide Schwalbe Marathon Extreme on the bike, that might have not had enough grip on gravel because of the tread design, the knobs are big and the grooves are small. Bike was unladen. There were several other mountain bikers (derailleur bikes) in the group I was in, they all pushed their bikes to the top, but I managed to pedal parts of that hill.