In another thread you started, I gave you the comment below before:
If I were to do the climb, I would use my current setup which is a 50/34 with 11/32 11 speed cassette. For a 20% grade, I would use the 34/32 and stand but position my body way over the handlebars with my quads almost hitting the handlebars and arms vertical and straight. I focus on staying on my toes and not collapsing the foot and push down while thinking to unweight the other foot - think running / sprinting. Keep the arms straight and do not colapse or pump them. This is the technique used in standing starts at the track where one needs a lot of torque from zero to low crank speed rpm. The goal is not to go fast just muscle up the grade. You may even be able to walk faster than you can climb on the bike.
The training for this technique is find a local steep hill and practice. Generally, my upper body fatigues before my legs. For me, climbing shorter steep stuff is a strength / technique versus aerobic matter.
And if all you have is rolling terrain, then use the power meter, (I think you have one), and hold constant power at lower cadence for 25 minutes. What that means in practice is that you will be in a big gear on the flats and a really big gear on any descents. You will need at least a 53/11. Or without a power meter, put the bike in the 53/11 or the biggest gear you have and muscle it around your typical rides. The proviso is that you work up to it so do not start out with 25 minutes of big gear work. Big gear strength work is hard on the back and knees but so will be the Six Gap Century.