I don't know the GT box but I do know that on the MK1 Uno the bottom and top shift gearboxes have a different fitting for the speedow cable, and are not interchangeable.
The MK1 top shift and MK2 boxes share the same cable, so perhaps if you removed a bottom shift box, try a MK1 top shift or MK2 cable. All Uno speedow cables I have seen are the same at the speedow end.
Which reminds me I have a Bottom shift speedow cable I no longer need.
I think the driven gear comes out of the gearbox if you unscrew an Allen-screw (recessed hex-head screw) in the diff housing below where the speedometer cable attaches. Then, a bullet-shaped steel collar comes out that retains the plastic driven gear and has threads at the top for the cable attachment. When you re-fit, be careful to align the hole in the collar with the Allen screw-pin.
The drive gear - more a ring that's part of the diff - may well be impossible to remove without taking the diff apart.
This goes for most FIAT gearboxes that I've experienced, including the top-change one in my Uno Turbo - but it may very well be different to yours - you never know for certain what came out of the factory that day...!