change gears on my giant trooper

[Pedroglobal]

Hey,
I got a giant trooper, with the gear ( Sram Twist Grip Rotary Handle DualDrive ) and cable broken and not working, do you guys know if its hard to change it myself? I live in amsterdam and in a shop is too expensive to replace the gears. Any thoughts about this? Where i can buy those type of gears? it is possible to change only the gears part that got the numbers?

When i change gears on gear level i see no diference, the gear doesn't change but the cable moves, this means perhaps that the problem is not on the cable, but on other part of the gears? on the photo 3 do you guys know if there is any part outofplace/ broken?

if needed i can take more photos!

Thank you guys!
Pedro