What gauge is a gear cable? 2mm? Are XT narrower on purpose? I can't get a brand new gear/brake wire to fit through the XT shifter, it's just too narrow. What's going on?

brake cables are thicker and have different 'stops on them' make sure it is a gear cable you are using and that you can see all the way through the shifter

Gear 1.2mm, brake 1.6mm

This is one of those double-ended cables. You cut one end off to make it a brake cable, the other to make it a gear cable. I can definitely see through the shifter, I've managed to poke a really thin wire through
quite happily.

I guess I'll go to the LBS and get some gear specific (narrower) cables…

Cheers.

You cut one end off to make it a brake cable, the other to make it a gear cable.

You sure they aren't both brake cable ends? One that works with vee brake levers (for example) and one that works with road levers (for example)?

You cut one end off to make it a brake cable, the other to make it a gear cable.

What you've got there is a brake cable that'll fit either MTB or drop bar levers.

Jinx! 😉

That'd be a brake cable with road and mtb fittings. It won't fit.

Thanks, all. Bought the right cables from the LBS, everything working perfectly now.

Cheers.