Most trucks have 5 gears in game as well as real life, it's not a problem since G27 have 6 gears and the Rear. But C-65115 have 9 gears and it's impossible to pass the 6th gear using the G27, how can I fix that issue?

I don't think you can. You have to be in automatic mode. That's why I've suggested the devs implement sequential shifting for the manual mode.