Hi Max, I am glad to see you here. I loved the 50mm picture you posted on Photo.net.
For the time being, I live in an equatorial climate with almost 100% humidity all year long, so I might not be able to provide helpful advice on an extremely cold climate like the one you'll be likely to experience in Ushuaia (are you going in Autumn?). But I have friends who regularly use Fuji G690 and GW690 for extended treks in the Alps mountains without any problem.
Usually, the first item to fail at lower temperatures is the battery. As the G690 can perform without any battery, it shouldn't be a problem. Be careful though with the AE 100mm, as the only speed available without battery power is 1/500. If it happens to you, you will have to take the battery out of the lens and put it into your pocket or glove until it is sufficiently warm to perform adequately.
Lubricant might also harden at freezing temperatures, so make sure that the focusing ring on your lenses is operating smoothly at normal temperatures. If not, have the lenses CLA-ed before departure. You could ask for lubricants designed especially for extreme temperatures, but I have heard that it is not advisable to leave them on the camera when it's used at normal temperatures, so you will need another CLA when you come back from Ushuaia.
Operating the camera with gloves should not be a problem, as all the commands are generously dimensioned, but I am not sure that loading 120 film in freezing temperatures with the wind blowing all over the place will require some additional training
Cheers,
Sebastien