It's actually several climbs of 6%-7% and one of 8%-10% with one pitch of 12%. There are apparently no 20% grades. I would climb all of those seated but try to stand for short periods every 10 minutes. Each of the 3 key climbs are ~1000'. I've ridden with 3 national caliber road riders who I'm sure could climb 1000' standing. None of them are 70. One of them was the fastest American RAAM rider one year. Another won National AG champ on his tandem and had several local LD records. The other was a 2 who was local crit champ for a couple years and still holds a local LD record.
If you go with gears than can't be turned seated, once you cramp, you're finished. Once you can't turn the cranks standing, you're finished. You have to have seated gearing.
The only way you can train for something like this is to experience gradually longer bits of it over several training rides. You need to be doing 1000' climbs, period. You have to learn the pacing, hydration, and nutrition that enable riders to repeat long, hard climbs. My last training ride started with a 3300' climb of 5%-7%. Went fine, but I've done a lot of that sort of thing. The issue with the 6 Gap is that there's almost no recovery between climbs.
I guess you have a Campy Chorus triple? Just put on a 26T in place of the 30 and experiment with cassettes to get the g.i. down to where you need them to be. A simple way to guess at where to start is to get on a 10% grade with your current setup and observe the speed you're making at the effort or power than you think you can use to climb 11,000'. Use this calculator:
Bicycle Gear Calculator
to convert that speed into gearing you can turn. I find 78 cadence is as slow as I can go and hold up for many hours. My guess is that you'll want a 26 X 30 or 32 bottom. The reason for 26 is that the smaller your chainring, the closer spaced will be the cassette, which will matter on long climbs.
There's also 1 descent of 12%-13% for ~2.5 miles, another twisty one of 5%-8% with a hairpin at the bottom. You're going to need good brakes and some pretty unworn rims if you're using rim brakes..