Over the years I've tested myself on some ridiculously steep climbing, both on and off-road; I have pretty good balance, but in this case, I don't ever remember it being an issue. I definitely used the 42 on the 17 percent section before the visitor's center and then any of the steep switchbacks on the gravel. I had to walk, once, on a steep washboard dirt section that was pretty soft right before the gravel ended. I could not power through it after six hours at nearly 12,000 feet. :) It was only about 15 feet. I had read about people who did it on 38mm tires with 30x34 and I was not going to attempt that. IMO you would need to be able to push at least 200 watts at9,000-12,000 feet to get through the gravel that way.
I was doing about 150 watts or so at that point (Stages right arm meter which reads 10-12 watts lower than my powertap).
After the dirt ends, there is a tiny break and then it climbs up through some brutal switchbacks at 10-13 percent; on the day I went there was a 10 mph headwind, so it was the only way I could breath and keep moving forward.

Also, I was running a 29x2.2 Speedking on back, so that is not quite as low a gear as on a road bike.