how do I train for a super-steep hill ?

[horizon]

I know it feels silly to spin a low gear and not make any effort seemingly, .

Assuminig the OP has any low gears. The factors are AIUI:

gears
gradient
weight of bike
weight of rider
fitness

Obviously one or more of those is out of kilter and indeed the OP may be always unable to cycle up this hill with those factors unchanged. The problem IMV is that the OP expects to be able to ride up and maybe on a bike that others can.

[Vorpal]

The gearing is a good point, and the OP may not realise the importance of it.

If they've got a road bike with high gearing, it may not be suitable for getting up a steep hill, even with interval training and plenty of preparation.

[ianrobo]

I decided long ago that my build is just not great for climbing. At 178 and 85-7kg I'm too heavily built and heavy to do it well, my power to weight ratio is appalling! I can happily ride for hours, I can ride up pretty much anything but not at anything resembling speed!

I am not much different in terms of build but learnt for rides in the mountains (Tiede I did 52km of climbing in 4 hours) it is all about keeping within a comfortable zone. So basically if you are unable to talk on a climb then knock it back, speed is irrelevant. For 15% and above climbs it does come down to brute force at time and as others have said only way to increase this are hill repeats.

I have a local one hill here 60m of climb in about 1.2km so 5% average but peaks of 10% which is perfect for it ... To train for climbs it is about keeping that Hr down.

[PH]

It's a 100 km ride with a hill we don't know the length of (Just that it's considered long) or just as importantly where it comes on the ride. If it's in the first half, save yourself, just get up it as comfortably as possible. If it's right near the end, flog yourself, give it everything you've got left.
Hill technique - there's plenty of it all over the net, a lot of it contradictory, all of it acknowledges that the more you do the easier it becomes. My favourite tip, I've forgotten where it came from, is to count the pedal downstrokes and push hard on the third - 1,2,3,1,2,3,1,2,3..... works for me, give it a try, the worst that can happen is it gives you something else to think about apart from how far from the top you are.