Don’t think you’d fall over because you’re going too slow. As said above, it then becomes a balance question, but typically, if you can still roll forward, you’ll not fall off. You’ll fall over when you run out of power, and can’t turn the pedals anymore. Running out of power has to do with pushing too high a gear for too long. So lower gears will likely help more than give you a risk of falling over.
I’d look at the whole problem a bit differently though - what type of hills do you expect to ride?
Gravel rides near me often include steep and/or loose stuff you’d expect to ride a MTB on. I usually feel under geared but I know I have to ramp up my fitness too.
I think Powerline climb at Leadville is a good consideration for this (not for gravel but on what a difference an easy gear can make). It’s steep and loose and even some of the Pros (Alex Howes) have suggested that they felt it was easier/faster to walk the steep pitches. Granted they are running like a 36 in the front…
I rode it at about 3-3.5 mph with the steepest portions putting me down at 2.7-2.9mph. On my 32 oval with a 52 that had me at cadences in the 50’s (with dips below 50) averaging 260-270 but requiring surges of 300-400w on the steepest pitches - almost 11 minutes. I don’t think I could have done anything more.
For pro reference, Keegen has the KOM did it in just under 7 minutes (average of 6.4mph) dipping to just below 4mph on the steepest pitches. Cadence dipped into the 60s and avg power was 340w.
All that to say I don’t think anyone has ridden up powerline and said their gearing was too light. 28/52 would be very useful there.
I ride in a lot of desert like terrain, so I can have perfect conditions for climbing where the rock is almost like concrete. This was my first time doing this climb, having no idea what I was in for, on my 40 pound MTB with 32x42 gearing:
