The Gas op does not move you at nearly the same speed as the rocket jumper at all. Nor does it provide nearly the same vertical capabilities.
When someone's trying to scam with the rocket jumper, they have significantly more options for exiting their cover. They can go to the left or right, they can go straight up. They can go up to the left or right. Gas op has a single predictable hop. If you have good aim you can hit them. And because of the cooldown, they can't just keep doing it. Rocket jumper is only limited by your health pool. In which, if the enemy can't touch your health anyway, then there's no concern.