The gearstick lever is made for 6 gears and does not have the function to shift the gearbox from 6-12 gears. However, if you have the ETS 2, you can select there how the gearstick should be. e.g. always skip a gear so that 6 gears become 12. another possibility is to set a button on the steering wheel in the ETS as a "gearstick". in this way, the game behaves as if the first gear were then the 7th gear. Unfortunately the gear lever alone has no buttons to assign.
I hope the answer is somewhat understandable.

I use the Force Shifter with the G29 steering wheel and so far mainly for ETS 2. I have assigned the L2 and R2 buttons for gearbox changers (with H-shift) (some heavy goods vehicles even have 16-speed gearboxes). Works perfectly.